I bottled half a batch a month or so ago with caps I'd been holding on to for around 10 years. a few days after bottling, my storage area smelled like a fresh ferment, and I figured I had a bottle bomb or two. Nope, nothing broken, after 3 weeks I chilled and opened a few to find them totally flat. The caps leaked.
A new bag of caps and carb drops and all is well. So, I guess there may be a limit before the liners in the caps dry out and won't hold pressure. BTW: they were Brewers Best caps.