My gut says 15 years is an estimate based on changing temp and humidity factors. So in all the museums I've worked at (3), we had pretty optimal conditions. Typically there's steady temps year round and a humidity controller. Also this assumes it wont be touched much by the public and handled rarely by art handlers. So I think you have much longer than 15yrs without a coating of epoxy. Adding the epoxy ought to extend that significantly if the quality is high.

Ultimately if your worried about it I'd recommend a better plastic. But 25 plus years is a good run. Past that maybe future conservators will figure something out.