Just used a ~4 year old refill card and got more than I expected!
Data point for those doubting that refills don't expire for a long time: I bought the $25 card 11/2008, used it now and it worked!
Also got ~218 minutes or a little more.
It actually brought my nominal minute rate down from ~12.79 cents to ~12.52.
I have Gold rewards on PAYGO.