Affiliate programs: How long do you keep your cookies for unique surfers?
 

I'm using NATS and I want to start a PPC program where affiliates get paid for unique hits. Trouble is deciding how long to keep cookies for unique surfers. The default for NATS is 1 day, so the same surfer could come back after 24 hours and hit the same banner and that will be counted as a unique hit again. Is the default one day to keep cookies for unique surfers a good time frame? Or should it be longer?

24 hours is pretty standard for cookies. (counting uniques)