I can't confirm this, but I saw this information posted by someone else...

The way it’s broken down is if you come with 500-800 hours it’s a 18 month contract as a captain. 800-1200 hours is 15 months. 1200+ hours is 12 months.

I believe this starts after you upgrade to captain and that you are eligible to upgrade once you hit 1200 hours.

I have been trying to confirm this, but have not had any luck yet.