I've never really thought too much about shelf life of seals and of course, you don't know how long the supplier has had them. In the aerospace industry (previous life), we would not allow any type of O ring (regardless of composition) to be on the shelf for more than 15 years and often far less. To be safe here, I would say 5-10 years as they may have spent some time at the supplier. You'll know by the feel of them if they are going off.
I doubt packing them in a vacuum would make a difference but I'm not a specialist in these sort of materials so, if anyone knows better I'd be interested to hear it. I store them in my workshop in a dark cupboard.
Any pictures of the crown you're trying to work on?
Cheers, Chris
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