How many hours a day should you practice?

I came across this article today, which is an excellent discussion on how much (and more importantly how to effectively) practice.

... a really good (and quick) read on how to focus your time and mental energy to get the best out of your practice - even when you don't have time for much.

The Bullet Proof Musician - How Many Hours to Practice

More isn't always better!  It's all about the quality of your work.