Back in 2010, I see JOhn Cookson commented that he stores his impellers in an airtight bag.
I do the same thing..ziploc bag, & also a scrap of rag soaked in MMO to keep it moist.
Thinking back, I replaced the impeller in the pump in Jan 2009 when I rebuilt the pump...so I guess I am 3 years or more as well..
Probably time to do it again.