More than 5, less than 50.  ;-)  

I think it's likely to happen before most of the people posting to this blog retire.  The airlines are in a death spiral right now.  It will only get worse as fuel prices rise, the economy falters, and travelers - business and leisure - cut back.

OK, another question:  Will the downward spiral to a state of demise be gradual to the end, or is some kind of jolting discontinuity foreseeable where all of a sudden the non-upper classes who are accustomed to flying relatively frequently if they wish will no longer be able to do so?
I think it could go either way.  Gradual is probably more likely, but I could see a situation where there's a sudden collapse.  War, resulting in fuel rationing?  An economic crisis, that forces us to choose between bailing out the airlines yet again, or spending on defense, food stamps, social security?  Disabling strikes or other labor unrest?