How long would a pilot shortage last? The length of the fastest zero-to-hero program.
There will never be a pilot shortage until the rules are changes.
There's no question that new pilot starts are down significantly, particularly when you consider the amount of Indian and Chinese students who are only here (the US) for training. In Palm Beach County alone, I have seen four flight schools close down in the past couple of years. My old flight school is still plugging along, but they are much slower compared to when I was teaching there. This is primarily due to cost, lack of loans, and terrible job prospects.
As far as the "zero-to-hero" program, I don't see how that can work very quickly with the new FAA rules for regional airline pilots. I know it's still up for debate, but at a minimum it's around 800 - 1,000 hours right? Plus there's still a possibility of requiring the ATP. Either way, these requirements will effectively eliminate the zero to hero pilot.
Will this equate to a pilot shortage? I think it might for some of the uber crappy regionals, IF they are forced to select from 1,000 - 2,000+ hr candidates in a normal job market. But if the work rules and pay are appropriate (starting pay at $35k, etc) with some of the better regionals, there's more than enough of us out in the CFI/corporate/charter/freight world that will be glad to jump in. But I sure as heck am not doing it for $20k/yr. That needs to change. Right now that will only attract the hungry, entry level pilot. There's plenty of those around, and always will be.
So will there be a pilot shortage? There might be one for experienced pilots, but never for low time commercial pilots.