I don't think I'd equate learning the guitar to learning violin. Most of my students really *want* to learn the guitar and play because they enjoy it. I'd imagine most students learning violin are doing so either because it's expected of them or because their parents are making them take lessons.
I usually tell my students to practice when they feel like playing. If you don't enjoy it, there's no point in forcing yourself to play. And if you do enjoy it, there's no point in forcing yourself to stick to a schedule that keeps you from playing.
If you enjoy the guitar and practice often, you'll learn to play better in a faster period of time. If you don't... you may be a great player someday but instead of it taking 5 years it'll take you 10 or 20.
Personally, I've never kept to any type of practice schedule... I don't think I've ever 'practiced' come to think of it, even when I was just beginning.
I just leave the guitar out ready to go at all times, and when I feel like it, I pick it up and play. I might be at it for 5 minutes or 5 hours... just depends on whether I'm getting into it or not.