From the FAQ

If I win a gift, when can I expect to receive it?

• Giveaway creators are allowed one week to send the gift after the giveaway has ended. Depending on the type of gift and the giveaway creator, you might receive the gift to the e-mail address you have listed on the account page, a key might be added beside the gift you won, or the giveaway creator might have sent you a friend request on Steam to make the exchange. Please remain patient, and do not contact the giveaway creator during this time. If the gift is not received after one week, you can mark the gift as not received, and attempt to contact the giveaway creator on Steam to follow-up.