If you are young and healthy, you can stay in there for 20 minutes with the water as hot as you can stand it, let it cool, and heat it on up again. If you are going to do that you should have a glass of iced water handy to cool your insides.
In the past I've let the water cool and refilled about 3 times. By the time I got out I was red as a boiled lobster...so overheated all I could do was grab the towel, lay it down on the bed and lie on the towel while the fan dried me off.