I'd hope it would last more than a couple of years! I've got a Garmin Edge 705 which I bought in 2008 and is still going strong. I haven't even had to replace the battery yet.

I'm thinking it's probably time to upgrade and was looking at the 520 plus. The unit is quite a bit smaller than the 705 but the screen is actually the same size. I'm a bit out of touch with the current market, though, so I'm not sure what there is at your sort of price.