Sometimes the crawlers need to revisit before they update the content of the ads being served. It partially depends on the popularity of your site about how often the crawlers visit. Once the crawlers update, the ads being displayed should be more relevant.
oh sorry im talking about my blog. i deleted a post yesterday (everyone in DP told me it's against TOS, though i still don't understand why, but just not wanting to risk), and the ads are now relevant to that deleted post. it's just that i spend nearly the whole day in front of the PC, so probably a day seem a very long time.