I wouldn't put a number on it, for example, new hires last year were told denver would probably be 5 years yet a lot got denver in a little over a year.
With the amount of retirements, more aircraft arriving new and used, an ever changing staffing model, upgrades galore, TK south closing, etc. you just don't know what could happen. All you can do is keep your bids in and see how it plays out.
Not sure if you are already in class but being on the bus or 737 gives you the option to bid up to the 756 right away vs. bidding down after 2 years on the 756 to go to 737 or bus.