The "life" of a MacBook is a vague thing, as for one person it could be a year, whereas for another it could be easily five to ten years. It really depends on the type of use--and abuse--it will get, and of course, your pocketbook.
For basic office-type needs, email, web, iLife-related stuff, and assuming it's treated with reasonable care, you could run it for several years with no complaint to the point where the OS and apps you want/need to run don't support your architecture. In that sense, resale value should be a non-issue.
If you tend to have a slightly higher-powered need, or the machine will withstand more physical abuse (as in it gets tossed in a backpack every day, you travel a lot, etc), two to three years at a minimum is still more than reasonable. Resale value should only be a minor consideration.
If you've got money to burn, you could just get the latest machine once a year for the hell of it, but there's really no good reason to justify that. Any residual resale value will still drop precipitously after each new model is released, so you'll still lose money in the long haul.
Remember, too, that in general, notebooks are not expected to have as long a lifespan as desktops because of the extra wear & tear.