Norway’s prison system is a very humanitarian system based on rehabilitation as opposed to punishment. The inmates are kept in what look like small dorms rather than cage-like cells. There’s stuff like therapy, job-training, etc. As a result, Norway has a very low re-offender rate.
The worst one can get is a minimum of 20 years. After twenty years, an inmate is re-evaluated to see if they should be released. Apparently, Varg was well-behaved and cooled-off enough that they deemed him eligible for release.
In regards to a freak like Anders Brevik. He’s very unrepentant and unruly and he slaughtered over 70 innocent kids. I doubt he’ll be released anytime soon.