I leaned of this technique for setting library hours at a math department party [1] many years ago. It is probably universal.
In this one and only section we offer two patterns for adjusting the hours of the campus library. They are:
1. Observe Usage
2. Open When Used
Students don't spend much time in libraries any more.
Therefore: Observe when students enter and leave. Keep track of what hours are busy and what hours aren't.
You will find that most students leave before closing. Consider adjusting hours to be Open When Used (2).
Your Obsered Usage (1) will show students go home early.
Who wants to keep the library open only when it isn't being used. In times of tight budget there is no point staffing an empty library.
Therefore: Close the library a half hour earlier. Be sure to post your new schedule.
New Library Hours
-------------------
10am - 2pm Weekdays
1am - 3pm Weekends
Once students adjust to your new schedule you will surely want to Observe Usage (1) once again.
References:
[1] Personal Correspondence