It will keep for some time but I think it has an expiration date. According to Radeon962 in
this thread that date is Feb 1 2013. Some research might be needed to verify that though. I never took the time to.
EDIT:I just sifted through my trash bin and it looks like Radeon962 was right. "This promotion ends on January 31, 2013; your product key must be activated no later than February 1, 2013." Copy and paste from the fine print in the email.