How long should we wait to pick a new winner for unclaimed giveaways ?

7 days.

You can i suppose wait as long as you wish but rules state you can re-roll after 7 days.So anything after that is up to you although i would do it asap.

Week is the standard amount. If you have tried contacting the winner and still nothing, then I'd ask for a reroll.

if contacting the winner by email or steam or by commenting on his previous giveaways didnt work and u have proof of sending the game a week ago [pref make screenshots ] than u can ask for a reroll on support ticket with reason that your unable to contact winner[only after 7 days though. If its a steamgift u can go to your steam press send gift again and u will see the option return gift to inventory [in case gift hasnt been activated] it gets trickier with keys though. A good thing to check is the FAQ too

cool thx people i was thinking the same thing myself but wanted it clarified :D i may give them till this week end then as its at a week now . if not claimed by then ill do a reroll .

If it's a key, make sure you checked to make sure they didn't activate it before you re-roll. If they have it activated, support can force-mark it as received.