I have a Microsoft 600 keyboard, which I've had for a couple of years now, and I was wondering how long keyboards last.
A good keyboard should last... a decade or more? A cheap one, maybe a year or two. Depends to some extent on use. I've had a computer since 1997, and I've definitely not had more than three in that time, and one of those was just because the PS/2 port my old keyboard used became obsolete and disappeared.
They're usually rated to a certain number of keypresses (the higher the better) and making sure you don't misuse them too much.
You absolutely do not need to spend much to get a keyboard that will last you years and years. I'd say about £20-30 (US$30-45) is quite enough. Mine's Logitech - keys are maybe a little on the stiff side for my liking, but otherwise been very happy with it: at least 5 years and still going.