How long do I wait?

Seven days is the minimum. Then you can request a re-roll.

Request a new winner after 7 days to SG support.

Is it a new user? Can you acced to their steam profile/mail direction? It depends on so many things.

According to the FAQ, you also have to attempt to reach the winner of your giveaway using e-mail and Steam, and provide evidence (like a screenshot) that you made such an attempt.

try emailing first - then steam - I emailed a few of mine to tell them they had won and they had simply forgotton - which happens I guess :)

Also a lot of Users are away for the weekend...I had quite a few GAs where I waited 4 days or so and then got a message saying: sorry...was away for the weekend thanks and so on
so patience xD

Seven days (7) but you could give him more time if you want...

for a reroll, i'm not sure 7 days is enough.
support will want proof you have tried to contact him. but i could be wrong...
try steam, make screenshots. if all fails....

but you can be nice and wait a bit longer, is the CV this urgent?

You're talking about your Dead Island GOTY giveaway, right?
If so, I hope you at least verified the winner to be from a supported region (the key should be NA+ROW, it won't activate in Europe).