Its valid for ever.
just click the app you want to buy-->click buy-->[you will have option here which display your credit card and the play store credit]
try it out with an app. unless you click buy it wont buy.
It's actually not valid forever. It expires on June 30, 2013 according to a link from Google. Let me see if I can find it.
Edit: https://forums.androidcentral.com/e.../intl/en_us/about/offer-terms/&token=XKLVoil6