Somebody posted this long time ago,forgot the link but most say shelf life of 2 years after that dis-guard and get new.
Well from a business/marketing standpoint they'd be fools not to put something like that on there.
It's just like paint. You can pull a can of paint out of your basement that is 20yrs old and if it has been kept in a reasonably well-controlled environment (not submerged in water, not frozen or heated to 150F) then the paint is absolutely fine to use provided you mix it really well beforehand.
Same with glue. I've got a bottle of woodglue that is probably 10yrs old and I just used some last night. I'm meticulous about keeping the tip cleaned up and sealed. Other than that the performance of the product hasn't degraded enough for me to notice.
If the thermal paste itself is good enough to be used as thermal paste for years on end (i.e. hot and exposed to atmosphere) in your rig then it darn well better be good enough to sit unused in a sealed tube at room-temperature for years on end too.