I was working on an 18-moth to two-year cycle with my desktop until I went with the dual-processor system. I've had it for almost three years now, and don't see upgrading for at least another. A lot of folks who run dual-processor systems say that they seem to feel useful longer.
Don't forget that for most dual/quad socket systems cost more upfront as well, so people tend to value the system more. eg motherboard typically 3-5x the cost, 2x for the CPUs, more for Reg/ECC RAM or FBDIMM's.
My personal 2CPU system is approaching 3yrs old, and will be keeping it for at least another 12-18mths. Cost in replacing it is admittedly one of those factors (new case + PSU, motherboard, 2x CPUs, RAM, gfx card, etc), however looking at what is out there, IMHO my system is still on par with most new entry/mid level single CPU systems in overall performance! (except for gfx, which is limited to a ATi R9600XT on AGP).
Overall my system is performing adequately for what I use my system for, so I don't see the need for upgrading. (It be nice to drop in a pair of Op275's into my current system, though).
PS. I do have a history of slow upgrades - 286@16 (~1991) -> P233MMX (1997) -> P4@1.6 (2001) -> Dual Opteron (2004) -> ??? (2008-09)