You'll be fine. The cards have a service life of several years (see above), and the balance does not expire. Do register your card at wmata.com, so that the balance is protected even if you lose the card.

Metrorail's paper fare cards, though, only carry value for a year or so; the mag stripe fades. After that, you won't be able to use them at the turnstiles but you should be able to trade them in for fresh cards (or transfer their value to a SmarTrip card) at a vending machine or at the Metro Center customer service counters.