Thinking about picking up a Nexus 7? You might want to get on that before the end of this month (right here), as the $25 Play Store credit promotion Google announced with its slate at Google I/O expires on September 30th. That means you must redeem the promotion before that date by signing into your Nexus 7 with a Google Wallet-linked Google account (that has a credit or debit card on file). Here are the terms indicating as such, emphasis ours:
2. In order to receive this promotional balance for the tablet activation, you must currently have or create a Google Wallet account on your tablet, and you must also add a credit card or debit card to such account (or already have a credit card or debit card in such account) for use in future purchase and payment transactions with the Google Wallet, by September 30, 2012. This promotional balance is not available for Google Wallet accounts where you only store your bank account or Carrier Billing information as a payment method.
If you already snapped up the promotional credit, you have nothing to worry about - at least for now. Just know that Google will get rid of any remaining part of that $25 one year from the date of the end of the promotion, on September 30th, 2013.