I am still confused about the upper bound, since release is far from guaranteed even then, but my wife tells me his release after 10 but before 21 years is solely at the courts discretion, while the psychologists enter the picture after 21. The Norwegian Wikipedia article on forvaring also states prosecutors can request the court extend the period when parole is impossible by up to five years as long as they do it more than three months before the stated period ends, and I would be stunned if they did not do so with Breivik.

As someone whose mother retired from a state Atty. Generals office and whose wife is a Norwegian legal assistant, I can tell you forvaring is very foreign to the American reference frame. Sufficiently so that I have been discussing it with my wife since before Breiviks murders in Oslo and on Utøya, and I still do not have a firm grasp of it. Breivik will not simply be released in 21 years though; the forvaring law was created about a decade ago to ensure that sort of thing does not happen, and Breivik will almost certainly be in prison for life.