Please read the site guidelines.

Allow the giveaway creator one week to send the gift. Please be patient and do not contact them during this time. If the gift is not received after one week, you may contact the giveaway creator on Steam to follow-up.

And the site FAQ.

Giveaway creators are allowed one week to send the gift after the giveaway has ended.

Please remain patient, and do not contact the giveaway creator during this time.

Key has been sent!

Please don't forget the following tips: If the winner is known before the gift/key has been sent, it is possible for people to impersonate them and steal their gift. This is why the winner is not revealed until you have ticked that you have received the game, and for that same reason you should never publicly say thank you or ask about the game until you have received it.

According to the site guidelines and site FAQ, the giveaway creator has a full week to send the gift/key before you should even attempt to make contact. After that, the guidelines and FAQ recommend that you try to contact the creator through Steam (again, using this method is to avoid giving away the winner's identity).