As much as you can stand would be the best answer, I suppose. There is a point sometime during a practice session where you stop moving forward mentally as you get tired. However, if you can stand it, that would be a good point to do some mindless exercising in order to build up your muscle memory. If you can play without tab, you will think of lots of things to practice after you get tired of sitting in front of a book and concentrating on tablature.
The answers to these questions of how often and how long are often left up to the student. You have to find what works best for you. For some, it may be a five-hour Saturday session once a week. For others, it may be 20 minutes each morning.
You should practice as much as you can for as long as you can and find what works best for your schedule and what is most productive. Determination and desire are the intangibles in setting practice time. If it’s fun, look out; if it’s chore, you’ll find the door. For those of you looking for a specific recommendation, I recommend practicing every day and at the time when you are most alert and focused.