It depends very much on how the patches are stored. I have a box of strips of patches that lasted about 10 years before the patches became unusable. But that was a whole box, in a plastic bag, in a cold dark place (a box of bike stuff in the garage). On the shop counter that same box would probably age out within 3-12 months depending on how often it got direct sunlight. What matters is heat, how well sealed the container is, and light (probably in that order). I think you're doing the right thing by inspecting the patch and deciding whether to use it.
That's not too useful if you're really asking "how often should I buy a new patch kit". In my experience at least two years, normally more like five. Again, I store my patches in a patch kit (plastic box), in a plastic bag with some tools, in my pannier. It's pretty cold and dark in there too. I find that the glue tube gets really gummy and useless before the patches wear out. But since I wear my tyres pretty much down to the canvas I go through a few patches most years, so that's not normally an issue. (I run the tyres until I get a puncture that seems to be attributable to tyre wear).
