You have to redeem it by March 5 but you can spend it whenever.
The promotion code ("Code") is good for one qualifying SEN account to redeem the $10 SEN wallet credit via the PlayStation®Store or Sony Entertainment Network services. Code expires March 5, 2013. Once Code is redeemed, however, the $10 SEN wallet credit can be used even after Code expiration. Code is not redeemable for cash, cannot be returned for cash, may not be used for any other purpose and will not be replaced if lost, stolen, or damaged.