My last computer was a 1Ghz desktop and it lasted 10 years (98 to 08). The OS was outdated and it was a pc, so it saw its share of freezes and crashes, and it even went down a couple of times for viruses, but it was still usable for web browsing, MS Office work and PDF reader, which is all I needed at the time - and I never upgraded the OS, always on the original Windows Millenium.
You don't have to update to the latest OS or apps to still use it. Many apps have backwards compatibility, so new machines will still recognize your "old" machine, and an OS will continue to work fine even after it's no longer supported for some time. If you really want to get the most out of a computer, use it until it no longer works: applications wise incompatible with current versions and cannot be upgraded, or are incompatible with the outdated hardware, or until it's so slow that one hour's worth of work/play time can be cut in half or more with something new.
My gf's Macbook is 4 years old and it still runs fine (and speedy) and fully updated with Snow Leopard and all updates (except iLife11, but it's coming soon).
Of course, it mostly depends on what programs you need to use. A professional that needs pro photo or video editing software will likely need to update their hardware more often than someone who mostly uses it for personal use (music, movies/shows, web browsing, light photography, Office tools, etc.).
