They tend to fail slowly. For example, I got a Carrier Air Wing that had a few random pixels around certain sprites but still totally playable. Within 2 years, the graphics were completely garbled and I had to swap the A board out for another. At the time, the seller said it was working fine. I do suspect that shipping these boards can trigger failure as well. For example, I just sold rsg a King of Dragons board that was working 100% fine as of a couple of months ago. In retrospect, I should have re-tested it but anyway it arrived suicided. Not only that, when I got it back the A board was also bad.
So basically, based on my observations, I would say if your boards work OK now and they are not subject to the rigors of USPS transportation, you're probably OK for the foreseeable future. That being said, failure is much more likely than your average PCB. The A board apocalypse has started.