They use to show up immediately, but I believe they now have a muti step process to activating ads on your websites. They will first accept you as a publisher and then give you the ad code to put on the site, but ads won't show immediately. They will then review you site to make sure it is ok and then proceed to activate your account fully or reject it. This process took about a week before my account was running with ads
My adverts started to show about 10 minutes after I pasted the code onto my website pages. At first there is just an empty space, but the advert will eventually show. I believe they use spiders to crawl your website so that the advert can be related to the site subject.