If they are sealed tight they should last for a long time. You may want to put just a couple drops of worm opil in with them so they stay slick. Plastic will absorb the oils that result from the cooking and can get sticky, but they should remain pliable for years and years if sealed up.
Keeping them out of direct exposure to sunlight helps too. Check the pegboard to see if sunlight coming thru a window hits the pachage. If it does, move them over a peg or six.