How long is the respond time?

no

the giveaway people have 1 week to give you the game @least, so without whining etc.

after that you can complain, and then if you dont get it at all you could also ask the admins to do something (i guess)

Sorry, i was kind of unclear, i meant if i (the winner) was unavailable.. lets say one week, can the giveaway-host decide that he will not wait longer and give it to somebody else / leave it to himself?

Minimum wait time is one week. After that, you can contact support to have the giveaway rerolled if you're tired of waiting.

If only there were some sort of FAQ to help out

I'm not sure but I think you have to be online so the person can trade you the game in Steam if he doesn't have a giftable copy. But if he has a giftable copy, you don't need to be online to receive it. And if it's a key, you'll probably just get it in your email!