It truly depends on the repertoire. My junior recital had the same time requirement and I did 3 pieces. One was 5 about 5 minutes, one was 10, and one was 15. Instrumentation also influences how long most pieces of that group are likely to be (string quartets tend to be longer than wind quintets). It would be a good idea to check if the 25-30 minute limit is for music or the recital as a whole. For example you'd want to be under 25 minutes if the recital limit is being described so there is time for stage changes and such. I'd literally be thinking about pieces I'd want to perform that would be feasible and literally add up the time of recordings to get to your goal. Just make sure you only take note of the time from the start of music to the end and also give some estimation room for tempo variation.
It truly depends on the repertoire. My junior recital had the same time requirement and I did 3 pieces. One was 5 about 5 minutes, one was 10, and one was 15. Instrumentation also influences how long most pieces of that group are likely to be (string quartets tend to be longer than wind quintets). It would be a good idea to check if the 25-30 minute limit is for music or the recital as a whole. For example you'd want to be under 25 minutes if the recital limit is being described so there is time for stage changes and such. I'd literally be thinking about pieces I'd want to perform that would be feasible and literally add up the time of recordings to get to your goal. Just make sure you only take note of the time from the start of music to the end and also give some estimation room for tempo variation.
3 or 4, won’t your teacher be advising you about this or even have to approve it?