I'm not convinced that hardcore practicing for 10 hours would be healthy or even beneficial, but I doubt Steve Vai is playing as HARD as he can for 10 hours straight every day.
One overlooked aspect to practicing technique is to play sloooowly.
If you haven't tried it, try playing a single octave scale over a period of about 10 seconds or longer. You'll be surprised at how difficult it may be to play every note as cleanly and clearly as possible! There is a lot of pressure to play fast, so we normally start comfortably and then try to build up to our maximum. Practicing EXTREMELY slowly will actually help build speed. It forces you to relax (which we should always do when playing), reveals unnecessary movement and effort in your technique, and can get you into a sort of meditative state where you block out everything except practicing the guitar. If you're into marathon practice sessions of several hours, it is very beneficial to do extremely slow scales and slurs periodically-- and sometimes for your whole session.
Also, don't discount mental practice. When you're in the car, lying in bed, in a boring meeting or class, etc. imagine playing perfectly-- your hands and fingers moving in perfect coordination, as relaxed as possible. Visualize your instrument in your hands and playing the passage or piece that you think you are unable to play. Also, if you're listening to music anyway, why not imagine yourself playing the piece? It has been said over and over in many fields that mastering something is 99% mental and 1% physical. You'd be surprised out how many things you can "figure out" away from the guitar by just thinking about it, rather than with your hands on it.
Between "physically" practicing the guitar and "mentally" practicing the guitar (I'm reluctant to even make the distinction), most of us can get 10 hours or more of quality practice in per day.
