In our area, the pros tend to drain and refill after each rental. I know that is a lot of work, but, given the way vacation users use the hot tub, to be fair to the next renters, you kind of have to.
They are not going to shower. They are going to add sunscreen, lotion, etc. Typically multiple users for longer times. And we don't want to talk about the activities. No way the bromine can keep up.