Year and a half upgrades (or less) according to Mesa's own sensitive nip, roid pumping, newhire recruiter:

"I am a Mesa recruiter, it's about a year and a half or less captain upgrade right now.. We have 750 pilots and we are hiring 800 in the next year and a half and that's not accounting for attrition.. Our most jr CA hasn't been hired yet"

~fozjared