If you are referring to the usage of points, they have no expiration date but the rewards program has a clause that they can terminate a Rewards account and forfeit the points in that account if there is no activity within a 24 month period, although I am not aware of anyone who has had their account terminated and points forfeited. Any other rewards (free night certificates, etc.) have an expiration date posted with them on you activity page.
You can see the rewards termination clause in Item #8 on this page:
Hope this helps!