You didn't say whether this was a commercial product or homemade. Commercial products typically last longer because they have the facilities to maximize the preservative processes.
Anyway, most food products start to diminish in quality after about 1 year. That's saying that the flavor, texture, color & nutritional quality will suffer after a year. As far as spoiling, the salt and vinegar in relish should keep bacteria at bay for awhile, providing that air isn't leaking past the jar's seal.
I wouldn't press my luck, though. The best rule with food is "When in doubt, throw it out".
