Breivik's killing of almost 100 people may earn him as little as 10 years jail time. Although the maximum sentence in Norway is 21 years (with prisoners normally released on parole after a third of this) there is a substantial 'discount' for a guilty plea; as this Domstol publication says " The criteria for entering judgment on a guilty plea is that the accused makes an unreserved confession supported by the evidence in the case. In such cases the sentencing framework cannot exceed ten years, and the accused must concur with the procedure. These cases are heard by a single professional judge."
However, there seems to be a further recourse called 'containment', used where a prisoner is deemed a danger to society with a likelihood of further violent crimes if released, a maximum period of 21 years can also be used, with a minimum time before parole of 10 years.
Brievik is 32. If he serves a full 21 years he will be out when he's 53 - with half a life ahead of him. I think this puts the rationality of his acts into perspective. Not a crazed man at the end of his tether but rather an evil man who plotted these horrors with complete rationality.
But even Brievik doesn't shake my opposition to the death penalty; even his life is sacred, and is not ours to take.