I've been in some rooms that are kept at 120 degrees and 80% humidity. You can work for about an 45 minutes to an hour at most (Adult male). Working being monkeying with stuff in an electrical cabinet.

I think you'd be in serious trouble after 2-3 hours.

Most saunas have relatively low levels of humidity, between 20% and 30%. It's hard to keep high humidity levels when temperature rises. Other than that, it's generally suggested to not exceed 20 minutes in a sauna. Steam rooms on the other hand have higher humidity levels but lower temperature and it's not allowed to stay more than 15 minutes inside.

From personal experience I've never used a sauna for more than 10 minutes and I always carry a bottle of water with me inside.