Mine are holding up pretty good. If in fact Hot Toys is using "pure" rubber.
Except for some "glue issues", they seems to be doing fine.
If it's silicone-rubber, the Aliens should last indefinitely. Silicone doesn't deteriorate.
Sorry to take some time to get back to you. Attempting to get a definitive answer for you, I contacted Dow Corning (big time silicone manufacturer). Here's what I asked:
"The [items]are made from silicone rubber. How long does this silicone last; does the silicone deteriorate over time"
Here's their reply:
"Silicone rubber has greater durability when compared to traditional rubbers and it also has more elasticity compared to soft plastics.
The breaking of the polymer matrix is the typical way in which the material degrades. Fillers and other additives that are attached to the polymer matrix can be the source or site of detachment. Whether by chemical exposure, UV breakdown or mechanical abrading, silicone is likely to outperform other materials."
Huh? Not much of an answer in regards to how long the silicone will last.
While I don't know how long the silicone will last, I do know that when the silicone comes in contact with [some]items made from rubber it causes a chemical reaction which will yellow.. clear silicone. For [example], SCUBA masks with silicone skirts should be stored in a separate case.
As for time, I can only speak from my own experience, but I have several [Scuba Masks] that are over 10 years old; I keep them in cases, away from [other] products, and the silicone still looks almost new.
Hope this helps,
Henry Babcock
AllExperts SCUBA Expert