If it goes like the first through third gen Core i series, two years. Haswell supports it, and Broadwell (the tweaking/ die shrink of Haswell due out next year) will probably work with it. After that, who knows - but Intel doesn't have a great recent history.
There also have been hints that Intel may be moving more towards BGA sockets, that is, CPUs soldered onto the board and thus non-replaceable.
To a large degree, I agree, I keep a high end 775/9650 rig, a 1155/2500K and an 1155/3750K around for support purposes and soon to add a 1150/Haswell for support/testing (think I'll prob drop the 775 soon and put it in with my loaners, sort of a shame, still a great rig, 8GB, runs at 4GHz