Our study hall hours were based on GPA.
All new members were required to do 10 hours/week for the first semester. No exceptions. All seniors in their last semester were exempt from study hall hours (unless their GPA was lower than 2.0).
It seems like the GPA breakdown went something like:
4.0- no hours
3.9-3.5- 2 hours/week
3.4-3.0- 4 hours/week
2.9-2.5- 6 hours/week
2.4-2.0- 8 hours/week
below 2.0- 10 hours/week
We had a book in the study hall room that had a page for each member. When she did her study hall hours, she signed in and out on that page. The study hall hours were proctored by scholarship committee members (or very well-trusted, responsible sisters). The proctor had to sign your page when you signed in or out of the study hall. That way, you couldn't cheat on your hours.
The scholarship chairwoman took the book at the end of each week and added up the hours for each sister. If you missed hours, you could make them up the next week... during dead week and finals week there were extra study halls, so lots of people made up missed hours during those times. She sometimes gave other opportunities to make up hours: for example, you could bring in old tests for our test file and that would give you an hour for each test...or you could do certain community service activities to make up hours. If you failed to make up your hours or if you got waaaaaay behind, you were fined $5 for each hour and you could have social privileges revoked. Also, the scholarship chair wrote letters to people's parents if they got really behind on study hall hours! Nobody liked that!! (I will say that the scholarship chair has to have a STRONG backbone... ours had to put up with a lot of crap from angry sisters.) But our chapter GPA improved.
If you had a heavy courseload and had a job/other responsibilities, the sholarship chair would often work with you...one semester I was allowed to do my study hall hours in the computer lab or in my room.
Good luck!