Thanks for posting this. My dad has been in molded plastics for like 40 years now and he would love to read this.Any one particular gun might last forever, another might fail much earlier.
There can at times be some variation in mold quality from piece to piece that will affect how long each gun can go before fatigue sets in. Minute variations in wall thickness in key places might be the difference between lasting 15k rounds and 500k rounds.
On Edit: here's a link to a BASF article on the fatigue performance of PA66 and PA6, which are are the most common resins use for a fiber-reinforced polymers. Most plastic gumns are going to behave somewhat similarly: http://www2.basf.us//PLASTICSWEB/displayanyfile?id=0901a5e18000488e
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It'll last at least until you come to your senses and ditch it for another 1911.
The Greenies (environmental movement) constantly tells us that plastic lasts forever—part of that "save the planet" stuff. Should be awhile before that plastic gun deteriorates—unless you bought a Glock cause those things always blow up in your hand after a couple hundred rounds.
Ultraviolet radiation breaks down the plastics as I believe oils and petroleum distillates will too.
Everything is biodegradable! Just give it enough time...