In my experience, LONG practice sessions are much more effective than short ones. E.g., I've found I get far more out of one 8 hour practice session than four 2 hour sessions.

I'd also say you need to do it intensely - you've got to be into and "feeling" what you're doing. If you're just sitting there waiting for your 3 hours of "practice time" to elapse so you can go chill you may as well just not do it.