The Norway example is misleading for people who are primarily familiar with US sentencing. Twenty one years is the maximum sentence one can be given initially, but it can be extended five years at a time at the end of the sentence. There are no limits on the number of five year extensions, so a twenty one year sentence can effectively be a life sentence.

It's basically equivalent to a US sentence of life in prison with the possibility of parole after 21 years, though the default is that you're released after 21 years, as opposed to the US default of you continuing to stay in prison unless you can convince a parole board to release you.