Consider the following 3 factors to determine how much YOU should practice:
1) Lessons don't make the musician. This isn't like school, where students mostly make progress when they're with the teacher. Music students get better between the lessons, not at them. For beginners and advanced students, children and adults, the best music teacher is largely there to guide the practice time and help the students know how to practice effectively.
2) Practice consistency is much more important than the amount of time. Creating music has more to do with repetition and muscle memory than just getting concepts into one's head. It's like memorizing the Declaration of Independence: it's going to take a lot of time and repetition. You can't cram for a big performance, and you have to keep practicing to maintain your ability. Better that you practice a little bit 5 times a week than get in a huge practice marathon once a month.
3) Goals guide the practice. Students should attempt meet the week's goals, not just log a certain amount of time at their instrument. This mentality will help make the practice time a lot more focused. You don't want to zone out during your practice time. Just like preparing for any test in a school, preparing to perform well will take more time some weeks than others.