I'm thinking of finnish-style saunas, which operate at 60 to 100 °C [1] (with 100°C being a frequent option). You usually stay in them 10-15 min at a time, but I guess if you keep hydrated you could stay a lot longer.

Finnish saunas are kept very dry. Sufficiently dry air makes very hot temperatures bearable because evaporation cooling from sweat gets more effective the dryer the surrounding air is. Water on the other hand makes sweating useless, which is why you can't tolerate water temperatures above body temperature (depending on how much of you is submerged).

1: https://en.wikipedia.org/wiki/Finnish_sauna#Types_of_sauna

"Air temperatures averaged around 75–100 °C (167–212 °F) but sometimes exceeded 110 °C (230 °F) in a traditional Finnish sauna." [0]

Apparently so! TIL.

[0] https://en.wikipedia.org/wiki/Sauna