Its an interesting question of how long the inhibitors last in undiluted, metal-free state.
Personally, I figure if its any older than the allowable service calendar life of the coolant, I toss it. IOW, if it is 5 year coolant, if unused but 5 years old, Ill toss it. Of course that's not hard and fast, and I might err on the side of caution too... But I figure I don't want oo old of coolant to be in the system at end of its life...