I strongly doubt that Breivik will ever be let out of solitary confinement unless the Norwegian high court finds that the Norwegian penal system violates Norwegian law. I doubt that even a conviction in the European court of human rights would cause Norwegian authorities to change the practice.
The official explanation for the solitary confinement is that other prisoners would kill him if they had access to him. The word punishment has never been been used to describe his extraordinary sentencing conditions. Actually, it's pretty ironic to have this discussion the same week that a convicted child sex offender was killed in a Norwegian prison after numerous warnings.
Breivik killed dozens of children of Arbeiderpartiet politicians, one of the biggest parties and the ruling party at the time. Norwegian society, great though it is in many ways, is still not capable of admitting that our relationship with the rule of law is somewhat selective.
Note: This is not in _any_ was a defence of Breivik's actions or ideology, rather a commentary on how legal principles are difficult for even a principled society to uphold in extreme cases. (It's also worth pointing out that Norway has been repeatedly criticised by the UN for our use of solitary confinement against arrestees that are not yet charged with a crime).