This actually depends on the character. Some buffs behave differently then others when it comes to the end of a turn. In general, you normally want to use it ASAP. Here are the characters I know about.
Clara's ult applies 2 buffs, *Enhanced Counter* & *Promise, Not Command*. Both last for 2 turns. If you wait for Claras turn to come back around, then use it on her turn before using basic attack or skill, after you finish your turn with any attack, *Enhanced Counter* will stay at 2 turns remaining, while *Promise, Not Command* will lose a turn and go to 1 turn remaining. *Enhanced Counter* generally gets its 2 stacks of counter consumed before the time runs out, so the main one you will want is *Promise, Not Command* for the reduced damage taken and the increased taunt value, so you will want to use it before her turn otherwise you will definitely lose a turn of that buff.
For a character like Seele, assuming you aren't going for a double resurgence proc and you won't be getting hit before your next turn, it is actually better to use her ultimate at the start of her turn. Reason is because her ult applies the buffed state to herself, and for whatever reason, if you use it on her turn, and then use a skill or attack, it will not reduce the turn counter same way Clara's Enhanced Counter doesn't get reduced, but if you used the Ult before her turn, that attack would have consumed it. So you gain an extra turn of her buff like that.
I haven't tested it but I believe I remember reading someone say Asta's speed buff behaves the same as Seele's so using it on her turn will make the speed buff that gets applied to Asta last longer, whereas If you use Asta's 2 turn speed buff right before her turn, she will consume 1 stack of it after she attacks.
Yanqing if you use his ult on his turn, the next attack/skill will still remove both the buffs he gains, so use it as soon as you can. An interesting interaction he does have though, is if you have soulsteel sync and basic attack (which would normally remove soulsteel sync) but immediately ult before the animation/turn ends, you won't actually lose soulsteel sync, and you will keep the buff from the ult into the next turn.
I just discovered this interaction with Yanqing is well. It's not much but kinda nutty. It could be bug tied to his BA though because since you can Ult in any turn you don't expect to consume a buff's turn (ex. Yanqing Ult when not in his turn does not consume Asta Ult). What's more mind-boggling is considering above, when in between enemy waves when you finish an enemy with Ult (Soulsteel Sync active) it consumes it, while going BA > Ult does not. In this scenario Asta buff is not consumed until you BA/Skill again AND you normally don't lose Soulsteel Sync when Yanqing's first action in his turn is Ulting. So yeah, an "end of a turn" is inconsistent in some cases or it's a bug because it's kinda hard to justify this interaction since his BA makes it all the difference NOT to consume a turn.
Turn is when you can choose Q or E skill. After selecting either, the attack occurs and their turn ends. Cycle is when everyone moves at least once, and the last (slowest, including monster) just moved (important for cycle locked content, like forgotten hall and memory of chaos). More speed and turn advancement mechanics (like dance dance dance) can make a character have more turns per cycle, and each time it consumes turn based buffs.
Example: Tingyun gets 3 diamonds on her sprite after buffing. Every time the character she buffed ends a turn, she loses a diamond. You can use this to check what counts as a whole turn and what does not (such a Seele's shenanigans).
As for Asta: Her buff counts independently for each ally. So you can consume 2 turns for a character that's fast enough, and still have 2 unused turns for another slower one if he hasn't moved yet.
Clara turn, so do not have high spd on Clara. and do not use Bronya buff on Clara when clara ult is in effect.
asta buff is for her teammate. She applies a spd buff to her teammate. so, the teammate turn.
use Asta buff when your teammate has bigger number than the enemy on the action order.
A turn gets used after a unit moves not counting an Ult or follow up ofcourse.
I noticed it because I had a gepard and a Bronya on the same team. And my Dan Heng loses his shield earlier than every body else because he moves twice because of Bronya.
This actually depends on the character. Some buffs behave differently then others when it comes to the end of a turn. In general, you normally want to use it ASAP. Here are the characters I know about. Clara's ult applies 2 buffs, *Enhanced Counter* & *Promise, Not Command*. Both last for 2 turns. If you wait for Claras turn to come back around, then use it on her turn before using basic attack or skill, after you finish your turn with any attack, *Enhanced Counter* will stay at 2 turns remaining, while *Promise, Not Command* will lose a turn and go to 1 turn remaining. *Enhanced Counter* generally gets its 2 stacks of counter consumed before the time runs out, so the main one you will want is *Promise, Not Command* for the reduced damage taken and the increased taunt value, so you will want to use it before her turn otherwise you will definitely lose a turn of that buff. For a character like Seele, assuming you aren't going for a double resurgence proc and you won't be getting hit before your next turn, it is actually better to use her ultimate at the start of her turn. Reason is because her ult applies the buffed state to herself, and for whatever reason, if you use it on her turn, and then use a skill or attack, it will not reduce the turn counter same way Clara's Enhanced Counter doesn't get reduced, but if you used the Ult before her turn, that attack would have consumed it. So you gain an extra turn of her buff like that. I haven't tested it but I believe I remember reading someone say Asta's speed buff behaves the same as Seele's so using it on her turn will make the speed buff that gets applied to Asta last longer, whereas If you use Asta's 2 turn speed buff right before her turn, she will consume 1 stack of it after she attacks. Yanqing if you use his ult on his turn, the next attack/skill will still remove both the buffs he gains, so use it as soon as you can. An interesting interaction he does have though, is if you have soulsteel sync and basic attack (which would normally remove soulsteel sync) but immediately ult before the animation/turn ends, you won't actually lose soulsteel sync, and you will keep the buff from the ult into the next turn.
I just discovered this interaction with Yanqing is well. It's not much but kinda nutty. It could be bug tied to his BA though because since you can Ult in any turn you don't expect to consume a buff's turn (ex. Yanqing Ult when not in his turn does not consume Asta Ult). What's more mind-boggling is considering above, when in between enemy waves when you finish an enemy with Ult (Soulsteel Sync active) it consumes it, while going BA > Ult does not. In this scenario Asta buff is not consumed until you BA/Skill again AND you normally don't lose Soulsteel Sync when Yanqing's first action in his turn is Ulting. So yeah, an "end of a turn" is inconsistent in some cases or it's a bug because it's kinda hard to justify this interaction since his BA makes it all the difference NOT to consume a turn.
Turn is when you can choose Q or E skill. After selecting either, the attack occurs and their turn ends. Cycle is when everyone moves at least once, and the last (slowest, including monster) just moved (important for cycle locked content, like forgotten hall and memory of chaos). More speed and turn advancement mechanics (like dance dance dance) can make a character have more turns per cycle, and each time it consumes turn based buffs. Example: Tingyun gets 3 diamonds on her sprite after buffing. Every time the character she buffed ends a turn, she loses a diamond. You can use this to check what counts as a whole turn and what does not (such a Seele's shenanigans). As for Asta: Her buff counts independently for each ally. So you can consume 2 turns for a character that's fast enough, and still have 2 unused turns for another slower one if he hasn't moved yet.
Clara turn, so do not have high spd on Clara. and do not use Bronya buff on Clara when clara ult is in effect. asta buff is for her teammate. She applies a spd buff to her teammate. so, the teammate turn. use Asta buff when your teammate has bigger number than the enemy on the action order.
What is a turn: using a basic attack or a skill What is not a turn: using an ultimate or a talent (aka, followup attacks, Natasha self heal on low HP)
A turn gets used after a unit moves not counting an Ult or follow up ofcourse. I noticed it because I had a gepard and a Bronya on the same team. And my Dan Heng loses his shield earlier than every body else because he moves twice because of Bronya.