My university just give us the total amount of hours we are to spend out on placement.
In order to meet our university requirements, we also have to complete 2 weeks worth of night shifts.
The rest of our hours are up to us to work whenever we can within the allocated placement terms.
I think like zippy said, it's all up to the university and what their policies and procedures are.
Some may say you have to work so many weekend hours, so many night shift hours and so many day time hours.
If your work are understanding of your situation though, hopefully it won't be an issue.
Or if you do something like getting on the NHS Bank, then you can work as and when.