I think that practicing 8+ hour a day it is possible, my teacher (former guitar player) was really bad when we started (we all were actually) bu the thing is when the other guitar player joined the band he learned everithing by him self (no teacher) so Jerzey felt... mmm.. bad I guess, so he started practicing bout 8 hours a day and in just a couple of months he wes getting up to speed with Hugo and eventually became even more "technical" than Hugo.

So my point is, 2...3...8...12 hrs... it's just a matter of what are you trying to achive, someone said "make a plan for your practice" in this post, you start maybe with independence and picking, then some scales, speed on scales, single string scales, multiple string scales, chords, maybe some listening (styles and stuff) and some writting.
If you put all that together ya may get to the 4 or 5 hours easy (without getting tediously boring in some parts).

And maybe I must stress the listening part.

A famous drummer called Adam Nussbaum once said "when it comes to technic, I don't wan't just fast hand, but fast ears"

Getting music in your head getting melody out of all that hours of practice it's the real deal (ok... shredding and noodling over some high gain power lick it's pretty fun too) but music should be the final goal, being able of doing it ripping... well.. that's attitude.