Ask your painter or read the directions on the paint can to be sure, but my thought is that if your bathroom is well ventilated and you towel off excess water in the shower as soon as you finish, you could use the shower 24 hours or so after the painting was done (assuming the paint is water based and not oil based). I have done so with no problems.

If you live in a high humidity climate and don't use AC, then probably best to wait a couple of days.

All water based paints will take up to two weeks to reach a full cure. So, the longer you wait, the better off you'll be. I always tell my clients to wait at least 3 days. Longer if its really humid. Anything less will lead to future problems.

If the painter applies all three coats the same workday, you are definitely headed for trouble. Let the primer dry fully. Let the first coat dry fully. Let the final coat dry a day before showering.
Each successive coat will entrap any offgassing and moisture still present in the previous coats, this rushing things along is the cause of many paint failures, by blistering and peeling; then the victims ask "wahappen?" on this board.
Casey

I just wanted to chime in for anyone reading this upon google searching like I was - I showered the day after the primer and two costs were applied and it completely ruined the wall. Painters had to come back and repaint all over again. The picture shows how it looked DRY 4 days later and why they had to come back. The painters didn't know how long I was supposed to wait so this time I'll take @randita's advice and use the other bathroom for as close to at least a week as I can.