How long can a 16 year old go to jail for mugging a 15 year old with a knife taking a $150 watch?

OK you are talking armed robbery with a dangerous weapon. The 16 year old will be treated as an adult, unless he or she is reverse certified back to juvenile court. It's a big person crime with a big person sentence most time. Not less than 5 years, no cap, and the serve 85% of sentence before parole eligible applies.