It's a combination of merit based mixed with seniority based. You need 5 Evals from Captain's recommending you to become an "A" SIC. Once an "A" SIC you need 5 more Evals for upgrade. After that it's into the pool for the Captain's board which is supposed to be based on seniority. Or that's the theory anyways...

You can make "A" SIC pretty quickly if you bust your butt and do the job right but after that you're at the mercy of attrition and growth. Though there's been a decent amount of attrition with people headed to the majors lately so who knows what the actual upgrade times are at right now.