A more detailed discussion from a forum is linked here:
But, in short, 85 degrees Celsius is where you need to start worrying and you have acceptable running temperatures. If you're in a really cold area and have a fan on it and it's running at temps like that, I'd venture to say something is wrong, but otherwise you're fine.
Also, people have tried to overheat the pi and failed.
by Gert van Loo » Wed Aug 05, 2015 7:03 am
I was responsible for the temperature testing of the PI-2.
This is always based on a few sample and only a short time but I did manage
to run a Pi-2 in an oven measuring the on-die temperature as 150C. But only for a short time.
It still worked fine several weeks later.
In fact I had some system crashes at lower temperatures which turned out to be the SD-card giving up
when the oven reached ~100C. I had to try several cards to see which one to use.