Once you have a requested a Steam Key, the key combination will show up next to the station you requested it for.
If there are no keys left available for the game it will show "Pending" next to the station. The content creator will be notified and once they upload more keys you will receive an email with the keys assigned to you.
There is not a set amount of time it takes to receive the keys if more have to be uploaded. It depends on how quickly the content creator uploads the additional keys.