As a law grad who's been through the same situation, I would advise you against putting a specific number on it; the hours of revision you do are irrelevant, it's all about how much you actually learn.
You might learn more in 1 hour of 'good' revision than you would in 5 hours of 'meh' revision (for lack of a better description). Personally, I always performed better with 3/4 hours of concentrated revision, spread throughout the day. I also revised in terms of topics not hours - i.e, I want X, Y, and Z done today - and found that this prevented me from being swamped.
The person who revises 2 hours a day might actually be learning more than the person who revises 8 hours a day - the hours are less important than the methods and strategies you employ.
TL;DR: the 'hours' are irrelevant - it's all about finding out what works for you and sticking with it. Your mileage may vary, so don't get hung up on what your peers are doing (especially as there's no guarantee they're learning more).
Good luck all!