Historically, the lifespan of a desktop is largely dependent on what it's used for. If it's not used for things that tax the CPU, a fairly low-end one is likely to serve your needs for up to 5-7 years provided you either don't skimp too much on the amount of RAM or add more later as warranted. And even then, it will probably still be functional, especially for basic uses like surfing and e-mail, just not as fast compared to what will be available in the same tier.