From what I understood from Sirius' imprisonment, life does mean life in the wizarding world. I assume that if there was some chance of Sirius getting parole or having a release date, it woul have been mentioned in one of the news articles. I live in New Zealand and whenever there is something on the news about a prisoner, the article always seems to end with " so and so would have been eligible for parole in 20??"
The important words (for me anyway) are can and life. Can is not will, so there is obviously a possibility for a lesser sentence. Life (in the UK, anyway) does not always mean life (as the Daily Mail never tires of reminding us). The sentence is often actually between 15 and 25 years