I think the high end cards will last as long as you're willing to tolerate them.
A few years ago I bought an X1800XT 256MB and an X1950XT 256MB (both Sapphire brand) a few months apart from each other. I also put Arctic Cooling Accelero X2's for both of them to help tame the temperatures and noise. The X1800XT lasted a a little over 6 months and had to be RMA'd to Newegg (back when Newegg accepted RMA's for up to one year from purchase date) due to the VRAM going bad and causing constant artifacts. The replacement lasted another 2 years in a friend's computer, as a "loaner"
, that eventually got trashed when he decided to finally upgrade.
I ended up using the X1950XT for about a year and a half before the same artifacting started to happen. At which point I just upgraded to an HD4850 512MB. Then a little after a year later I switched to two HD4850 1GB cards in Crossfire. A few months after that an HD5970. A few months after that a pair of GTX 570's. And at the end of the Step Up period, I'm currently now on my GTX 580's.
This goes back to my point that any tech will last you as long as you want it to. With the help of Ebay and the For Sale forums, it has become very easy to offload older hardware and upgrade to newer tech without taking as big of a financial hit. That and the fact that near high-end performance can be had for much cheaper price points today as opposed to what was available just a few short years ago. Thank you competition!