Well, as the Avada Kedavra, the Crucatious curse, and the Imperious (please excuse my spelling) are unforgivable curses, I assume using them would mean life imprisonment. Why don't you go back to GOF and ready the chapter when Moody/Crouch is teaching the class about them and see if he mentions anything about prison sentences because I assume he would. I think the fact they are Dark Magic as well would mean they would probably mean life imprisonment if caught using them.