That's going to vary from hospital to hospital and even specialty to specialty.
Main OR is required to take one 8 hour shift of call per week. It's sign up on a volunteer basis; some weeks people have no call because there's more trying to sign up than call available. Weekends are assigned- a combination of three shifts, some work some call. Every 3rd weekend.
Ortho takes call on weekends only, but may have to stay late to finish cases. Every 4th weekend.
Hearts are the big call requirements. These are either 16 hour shifts Mon-Thurs or 64 hour weekend shifts. Last paycheck, there was an average of about 20 hours of call back per team member.
Per state law, we can only work 20 hours in a 24 hour period and then must have an 8 hour break. My facility is pretty good about either giving people the day off or getting them out early if they've been in on call the night before. Those who reach the 20 hour limit are off no matter what.
Variations between facilities:
Is it a trauma center that is staffed 24/7? This may reduce the actual call back time as staff is in house to handle what comes up, and call people are used only in times of emergency.
Is there a second shift? This would reduce the need to stay late to finish cases.
What is the case volume, and when do most surgeons operate? Getting a small number of cases finished that start early would reduce needed call staff after first shift leaves. However, a lot of cases that start later would mean more cases running after first shift finishes.
These are questions you would need to ask the person interviewing you if they don't tell you off the bat.