well i have a doubt.i think he spent twelve years in prison.*grabsPOA book to check it.well as sirus said in the books the dementors could not suck away the thought that he was innocent.and that helped him to keep his sanity.he escaped from askaban as he was an animagus.and the dementors could not feel him when he was a dog.that was one of his major strength.and he was also very brave.he roamed about in hogwarts searching for wormtail as a dog a none other than lupin knew that he was an animagus in the school.
well thats all ive gat to say and
~~~~whoohooofirst to post!!!!!!