There's a hump around 2-3 minutes for myself. Once I get past that my skin gradually stops feeling like it's on fire and I can go for 8-10 minutes easy.
First time I went at a sauna/spa place, it was a big deal to watch someone grit their teeth and go into the cold plunge pool, and then step right back out.
I went more recently and there were people just hanging out in it like a hot tub. Granted, it was a group and they were only partially submerged. But they were in there for more than 30 minutes and no one wanted to go into the small plunge pool while they were there.
