I've had these lurking around my workshop for some time and thought it'd be interesting to see a side by side comparison of a prosthetic stored properly and one stored very improperly.
The white-ish one was obviously stored properly, the one that looks like overbaked bread was not. These were both made around the same time, give or take a couple weeks, approximately 3 and a half years ago.
The one on the left is still totally usable, the foam is slightly stiffer (but then again it could have come out like that in the beginning, I don't recall) but otherwise totally useable - in fact I'm planning on using it tomorrow perhaps. It was stored in a sealed bag away from direct light.
The one on the right was left in the back seat window of a car, during the summer, for maybe 7 months or so and has just been left out ever since. It's incredibly delicate, in fact a few tiny pieces crumbled away just picking it up to do this shot. It's totally stiff, the underside (not in the sun) is a yellow-ish colour and you can see the topside is that golden brown (slightly burned) bread crust colour - although totally rigid and a little shiny.
This is a good demonstration to customers who by prosthetics who wonder why things need to be stored properly and how long they can last. In a sealed bag (everything prosthetic piece I sell comes in a zip lock bag anyway) out of direct sunlight - and definitely not in the back window of a car for several month during the summer.
As I said, it's been about 3 and a half years and the piece is still totally useable. You hear some people go on about how foam latex (and latex in general) will degrade over time, it will, but it will take a LONG time, so there should never be any concern about buying a prosthetic piece like this and no using it right away - just store it properly and you can still use it years later.