I am a norwegian, living in Oslo.
even though our maximum prison sentence is 21 years, it is also possible for the court to give an extra sentence called forvaring (confinement - also in prison). This is not time limited, and is for people who represent a continued danger to society. It can be renewed for five years at a time, and can, in principle, be a life sentence.
I imagine that is what will happen to this man, as he clearly was not psychotic when he planned and executed the murders, but at the same time i cannot see how he could possible be rehabilitated. I imagine he will stay in prison for the rest of his life, or until he is an old man.
I am glad that we do not have capital punishments in Norway ( or lynchings). Look at McVeigh, the man who bombed Oklahoma city. The americans exectuted him, and that just made him a bigger hero in his circle. He was clearly pleased to be executed and in that way become a martyr for his cause. Much better to keep them in jail.
I am proud that our prime minister says that the way to honour the dead and fight extremism is to continue to be a democracy. If not he will have destroyed our country.