It is not a silly idea but, as has been said, it wouldn't work fairly. Some people, like me for instance, believe that anyone who invades the sanctity of someone's home with burglary can rot in jail for the rest of their life, which isn't fair ofcourse. I would however seriously support an alteration to this ridiculous 'ten years but out in 4 for good behaviour' lark. If someone gets 10 they should do that, and bad behaviour while incarcerated should attract additional time.