Sometimes practice is about what you feel comfortable doing....remember if you overpractice you can do yourself damage rather than good, and whats the point in that? You'll hear the pro's and some non proffessinal players say they do ****loads each day but that doesnt mean they will spend the whole time shredding and tiring themselves out. They could been looking at books and theory for some of that time, or maybe spending time just picking a few notes for a couple of hours, and they can be writing music down too.
One other thing: dont just practice exercises once you get into the art of the guitar. If you know enough to jam, then jam away.....otherwise your solos and riffs can sound like mechanical, rehashed versions of book/speed-exercises.
Is everyone capeable of playing like Vai etc......? I don't believe so, because some musicians would not have the discipline to reach such a standard, although that would not mean they had no talent. I'm inclined to Vai's viewpoint that everyone is unique and individual and with the right approach that can come out in playing - for some people playing like Vai, Satch etc would be the wrong approach to expressing themselves.
'There's no such thing as bad weather, there's only the wrong clothes...'