Link: How Many Hours a Day Should You Practice?
This is an excellent article by Dr. Noa Kageyama addressing this very question:
In addition to answering this question, it offers an excellent discussion on making the most of your practice time to improve your playing.
If you get a chance, check out his full site, also. He has lots of good advice on how to work with and overcome performance anxiety, stage fright, and personal anxiety about your musicianship.