The domain is the numbwe of hours he works. That means this will be the interval of hours that he can work which range from 0 to 24 (in the extreme case he works 24 hours in the day). This is because we are not told how much hours can he work as a waiter.
So the domain in this extreme case is:

If we assume that he has the limitation of working 8 hours, then the domain would be:

But the limitation for the domain is the amount of hours that he works, and that he cannot work more than 25