Mass murderer Anders Behring Breivik was handed the maximum sentence on Friday after an Oslo court found him sane and guilty of "acts of terror" for his Oslo bombing and shooting massacre that left 77 people dead in July last year.
A Norwegian anti-terrorism law from 2009 raises the maximum sentence to 30 years but the new law has yet to take force.
Outside Norway, Friday's sentence was seen by many as mild.
But Breivik's imprisonment can be extended indefinitely as long as he is considered a threat to society.
"It reflects Norway's culture. The goal of the judiciary system is to ultimately have a rehabilitation of criminals," Jo Stigen, a criminal law professor at Oslo University, told AFP.
He said most Norwegians do not think the legal system treated Breivik mildly.
"It's psychologically satisfying that he got the maximum sentence. That's a strong signal to society," he said.
A poll in daily Verdens Gang on Friday showed that 62 percent of Norwegians are convinced that Breivik "will never be a free man."
If that turns out to be the case, Breivik would be a very unique inmate in Norway. According to Stigen, no Norwegian prisoner has currently been detained for more than 21 years.
Another Oslo University professor, Hans Petter Graver, said however it was possible Breivik could walk free in less than 21 years.
"The main principle behind the Norwegian system is not for people to spend their life in prison but for them to be reintegrated into society," he told daily Dagbladet online.
"Nobody knows how Breivik will have evolved in 15, 20 years.... Society also evolves over time," he added.
Norway abolished life in prison in 1971, adapting its legislation to reflect reality. "Lifetime hadn't been used for a long time," Stigen explained.
The country then had to reinstate life sentences for crimes against humanity and genocide to respect its international obligations, though no Norwegian has yet been prosecuted for those crimes.
Breivik's massacre has not sparked a debate on the death penalty in Norway, where the last case of capital punishment dates back to 1948 as part of post-World War II executions, according to Amnesty International.
For civilian criminals, the death penalty was abolished in 1905.