I'm pretty sure there's an ABA rule against working more the 20 hours per week your first year. Under 20 hours (if I'm right) is therefore permissible, but never recommended.
That said, there were students in my class that worked throughout law school. It can be done.
Be wary of relying too much on firm connections. It's definitly good to keep up with your employer and hope to return, but in my experience law firms really only care about grades (they'll tell you otherwise, but ask the job seekers). It's amazing how some of these firms won't give the time of day to their respected former employees because they aren't in the top x% of the class.
A lot depends on how you manage your time and how efficient you are. It would help you a lot if you happen to be a fast reader or if you have some time at work that you could use to study. I can't give you an hourly figure of how much work there is in law school - it's different for everyone.