Silicone Seal Life

Good Day

I need to find out the expected useful life of silicone seals that are used on intercoolers for a mulitstage air compressor. The manufacturer, Basco did not specify a life, and just told me when i asked, that it should be changed whenever we pull the intercooler apart. The reason I need to know, is because there was one seal that was never changed for 6 years, and when we took the intercooler apart, the seal was brittle. It was not leaking before.

Can anyone point me in the right direction of getting information about the expected life of silicone seals.

Thanks