FedEx is already retiring 727-100's as their heavy maintenence check comes up. I think the 200's might be around awhile. We own some of the last ones made. The big questions is, what's going to replace it? There has been talk of 737-800's or just expanding our current Airbus fleet. I've heard that the A310 cost the same to operate as the 727. To answer your question though, I think FedEx will have 727's in their fleet for awhile.
The 727 will last a very very long time (as if it already hasn't) should the airlines decide to keep them. UA, AA, CA & USAir have retired theirs leaving NWA & DA as the only major passenger carriers operating them. They have a lot of life left with charter operators and freight carriers. A running joke at a major I'm familiar with is the 727 will be used to pick up the Airbus crews from the desert when they retire the A320's.
Maybe someone needs to start a "classic" airline. Must use old DC-8's, 727's, 707's etc. For the feeders we could use DC-3's, Convair's, and feed them with Heron's, Dove's, and Pilgrim's. All FA's must be able to conform to the requirements of the 50's and 60's. An oil shek would be the required first investor.