While I would surely agree that the minimum and medium security institutions in Scandinavia are certainly, by US standards, pretty easy time, no maximum security institution is a cake walk, no matter where it is.
Plus, like pretty much all prison systems in the world, there are significant gang issues in the prisons, and of course those gangs all fall along racial/religious boundaries. Brevik gets sent to actual prison and he will have to deal with the Muslim gangs, probably by being welcomed into the Aryan ones. This is a prisoner who will simply have no choice but to get heavily involved in gang activity in prison, which is pretty much a gift-wrapped invitation to extend his sentence indefinitely beyond 22 years. Not that I think for a moment he would actually survive two decades. That's a long time to be watching over your shoulder 24/7 for a shank with your name on it.