Just came across this interesting article. It might be of some help to you guys. How Many Hours a Day Should You Practice? | The Bulletproof Musician
For more guitar specific practice tips, check out the initial article on "John Petrucci's Wild Stringdom". It talks about how to divide your practice time into sections and how to focus on different aspects of playing guitar.
The same concept is also mentioned on some site. It was an interview with Steve Vai. Who gave few examples on how to practice...Was really helpful!!!