The number seems to come from Wikipedia[1]:

Prisoners sentenced to life imprisonment serve an average of 16 years, more for cases considered to be particularly grave. The only example in modern times of an individual serving significantly more than 16 years in prison is Palle Sørensen, who served 33 years for a quadruple police murder.

So I guess it's a parole, then (which cannot happen until 12 years into the sentence).

[1] https://en.wikipedia.org/wiki/Life_imprisonment_in_Denmark