He got 21 year special detention (forvaring), meaning he'll probably sit for life or until he is no longer deemed a threat to society. The detention will be renewed every 5 year or so by a special court.

But this is also problematic as the prisoner is locked up for an undetermined amount of time, which is suffering in and of itself. But I don't think it's worse than life without parole, which seems like absolute torture to me.

BTW, after 2015 the maximum sentence in Norway is 30 years.