RDK - airlinepilotcentral.com has the number of pilots at each airline and the number of retirements by year. We're currently experiencing a large number of retirements in the U.S. airline industry and the number of retirements is increasing.
Life is based on making reasonable assumptions. The assumptions of 2014 indicated, based on retirements that no one has been able to avoid since 1903 since all pilots 'age out' eventually, was that airline pilot upgrades at the largest U.S. airlines would be in the 6 yr (+/-) range. That's happening now. Based on the increasing number of retirements that will drop to 4 yrs (+/-) assuming there's no catastrophic economic event. But even that would only delay the upgrade process slightly.
You mentioned that you're a 737NG FO. Typical new hires have 5000-7000 hrs TT and 3000-4000 PIC, and usually 1000 hrs U.S. 121 PIC time. If you're serious about getting into the U.S. major airline pilot job market flying more as an FO isn't increasing your competitiveness as much as coming to the U.S. would. That would mean getting a regional job, upgrading in 2-4 years, getting 1000 hrs U.S. 121 PIC time, and hoping that you get the call.
Having 737NG time doesn't make a candidate more competitive at an airline that flies 737NG's. In the U.S. they don't care. You're going to the bottom of the list and what you get assigned will be based on you seniority within your new hire class (typically done by age) and what aircraft are available. A class of 20 might have 10 guys with 737 time and 10 guys with Airbus experience. But the 737 guys are senior and want to get assigned to the bases where the Airbus bids are. Guess what, 10 737 qualified guys will be going to Airbus school while 10 Airbus guys will be going to 737 school. It doesn't matter because every new hire will be going to a full school even if they have 10,000 hrs in type as Captain.