According to Mesa | Audries Aircraft Analysis, attrition and growth should maintain a similar upgrade time for the next year or two, after which it might go to roughly three years upgrade time.

That only accounts for upcoming deliveries. Should there be more growth and Mesa can sustain the hiring (they've done a good job of that so far), it's anyone's guess. That's my analysis anyway, but if anyone disagrees I'd love to hear their projections as well (genuinely).