How long before I can regift?

You are allowed to gift after that, since they are at fault for not taking the gift after the period of time.

Give someone else the gift if he doesn't claim it :P

You have one week to send, so it's assumed the giftee (is that even a word?) has one week to redeem. You can ask for a re-roll of the giveaway... don't know if there's a way to cancel the gift you already sent?

after 7 days from the end of the giveaway, you can reclaim it. Go into your "manage gift and guest pass" section, find the gift that you sent to him. It should say "Sent" but not redeemed. You should be able to double click it and unsend it or send it to your own email address without redeeming it.

Wouldn't you get a "gift not recieved" or "waiting for gift" status on SG?

IF you do decide to hold another giveaway, make sure you resend the gift back to yourself so the guy can't redeem it mid-new giveaway.