I can tell you that United is extremely short staffed. I can't speak for Delta and American, but I'd expect American to get very short very soon. And now that there are retirements, the majors need to fill their training pipelines. Those training pipelines are currently empty. I'd ballpark the training pipeline for United, American, and Delta at 500 per airline.

Yes, the industry is extremely cyclical. But right now airlines are running very lean and not adding capacity. We're on the upside of the cycle, not the downside.

Unless you think that the economy is about to collapse and/or the retirement age is about to be raised again, it's probably fairly safe to say that the hiring will last for at least the next three years. ... I say three years because I don't see any economic collapse or age change before that timeframe.

I think what you're going to see over the next few years is the opposite of what has happened over the last decade. The regionals will shrink and the majors will need to grow to fill those routes that used to be flown by the regionals. It won't be a 1:1 ratio; you'll see one 737 flight replace two RJ flights.
Even with attrition at the regionals, there will likely be stagnation there due to the regionals shrinking because there will be so few qualified newhires that the regionals will hire because they won't be competitive.