What are the fastest promoting rates in the Coast Guard, and how hard is it to make E-7?

My friend's little brother is enlisting as an OS. He said he didn't care what he did, and the recruiter tried to talk him in to FS. Luckily he changed his mind. I actually thought about joining the Coast Guard when I was his age. What is the average time frame to reach E-7 in OS? Also how hard would it be to reach CWO or E-9?