If you are going to work on N-reg aircraft then the ONLY license you can use is an A&P. This covers all disicplines required of a line engineer. If you want to work in a shop repairing boxes then you would need a repairmans certificate. This does not give you any entitlement to sign-off an in service aircraft. So if you want some advise, get the A&P. How it will take you will depend on your previous experience. That means anything from 10 days to 3 years if you are a complete virgin. Just keep filling out this OJT sheets!!!