My particular character has not been to school from the time he was 9 years old to the time he was 18. The reason for it is very long (and very illegal of those who kept him away.) How much time would it take for him with a couple of private tutors to learn enough to pass the GED (the U.S. test to pass out of high school and go onto a college or university)?
Edit: He's not a genius, but decently intelligent and creative. He's smart enough to craft some street poetry and how to open non-computerized security systems or measures (safes, car doors, some fences and some doors.)