My rule of thumb is similar to the homework rule of thumb (not that we actually have hw, since ours are homeschooled--or maybe it's all hw!): under six years of age, 5 minutes per instrument per day, or however much can be tolerated and still be fun, whichever is less, 6-7 days a week. Beginning with your sixth birthday, 10 min x 6-7 days, incrementing by 10 minutes with each succeeding birthday.

It helps when practice is broken down into sections targeting different skills, studies/exercises, or pieces. We also use a countdown timer. They are allowed to pause the timer for breaks, or to mess around on the instrument, as long as 80% of each practice session is spent on serious practicing. We had to add that recently, as ours would spend all of their time jamming if allowed, and claim they had practiced.

...pronounced like the long vowel and first letter of the alphabet...