Practice for adults is very different than practice for children ; so let me divide this entry into sections :
Kids : I would say any child up to 7 years old should practice 20 mins at a time . I would hope for every day or every other day . I also think lessons should be kept at about 30 mins long at this age group. This age group does not retain information well, so revisiting often helps with them to retain information they have learnt, but also stop them from forming bad habits when they can’t remember what was taught in the lesson . It’s also more cost effective as the teacher doesn’t need to spend a lot of lesson time correcting the student from mistakes learnt during their time away from class .
7-12 year olds I would expect lessons to be longer and therefore practice to reflect that , so up to 30 mins a day or every other day again.
12+ I would increase lesson length here as concentration levels tend to be good , so 45 minutes practice would be great to cover everything (every other day is ok as this age group retains information better than previous groups ).
Adults : I would love for adults to practice 30-60 mins at a time to make sure they have retained everything they have learned but also as most adults have concentration for about 60 mins it’s great to push yourself to that limit and beyond. Its such a wonderful feeling when you can play music for a full hour, particularly when you have just started learning as the sense of fulfilment is huge and very rewarding !