My understanding is that they'll generally keep you if the experts have good reason to believe you're still a threat to other people, otherwise you get pardoned at some point.
There aren't very many of these sentences, I think you can find the complete list here in the Danish article on Wikipedia:
I thought I'd never hear of a country with sane incarceration policy - by golly, don't imprison people past the point where they are dangerous, because then you are just spending money on pointless incarceration. Go Denmark!
There aren't very many of these sentences, I think you can find the complete list here in the Danish article on Wikipedia:
https://da.wikipedia.org/wiki/Livsvarigt_f%C3%A6ngsel
About 50 since 1971.