Peak mandatory retirements - which are really the only openings at the majors you can really take to the bank will occur in the 2022-2023 timeframe. The real question, IMHO, should be 'what can I do to be sure I am competitive for a call from the majors by 2023.'
What that means is that you should plan to get your ATP rating minimums as quickly as feasible, then go to whoever will fly you the most and upgrade you the quickest because to be competitive you are going to need:
1000 hours of Second in Command (FO) 121 time in a turbine powered aircraft.
1000 hours of Pilot in Command (Captain) 121 time in a turbine powered aircraft.
A four year college degree.
Yes, there will be guys and gals that get the call with less than that, but there will be a lot more guys and gals who will not get the call until they have MORE than that. Those are just going to get you in the ballpark. Other things, like airline trainer and check airman jobs would certainly be pluses, but at a minimum you want those three things and you want them by the retirement peak in early 2023.