Each player is different, but the general rule of thumb is that the closer a players GPA is to 2.0, the more study hall time he'll need to stay eligible. Also, the more important role he plays on your team, the more you'll want to try to ensure he makes the 2.0 GPA cutoff. Heavy study hall minutes do NOT guarantee a player will be eligible. It just improves the odds of eligibility.
You'll receive mid-term academic progress reports on each player, so it may be necessary to adjust study hall minutes at that time.