Does it take time for AdSense to show relevant ads?

Does it take time for AdSense to show relevant ads? This is a question, which turns up over and over again in AdSense related web forums and since it popped up verbatim today under "search engine queries" in my webservers statistics, I might as well write a quick article about it.

The short answer is: Yes, it takes time till AdSense delivers relevant banners to a new website. Of course, this answer automatically begs the next question, which is: "How long does it take?". This one however, is a little bit more difficult and requires some understanding about how ad placement works. The most important thing to keep in mind here is, that AdSense is keyword driven. That means, the more text there is on a page, centering around a certain topic, the easier it is for Google to pick the right banners.

So, how does banner choosing work in a general context? Consider having just registered a new domain and uploaded a single HTML file to it, containing an article and a banner (note: You'd likely not get approved for AdSense with just one page of text, but for the sake of the example, lets assume you did).
Now you view that page for the first time in your browser and Google will be completely baffled by someone requesting a banner for a page, it never heard of before, hosted on a domain, it never heard of before as well. Which type of advertisement would be fitting? No way of telling at this point, but since something has to be delivered right away, the system falls back to the safe choice, which is showing a public service ad (PSA) and then telling one of it's spider to have a look at that wonderful new business opportunity.
The spider now, being a good employee, will let the system know, that it is currently awkwardly busy evaluating other sites, but promises to have a look as soon as it finds the time. In the meantime, this means more PSA (and therefore no earnings) for you.

Eventually, the spider will visit your new page (you know when it did, as soon as your webserver log shows an entry from a Browser identifying itself as "Mediapartners-Google"). From here on, there are two options:

1. The page makes it very clear to the robot, what it is about.
2. The page is beating around the bush.

In the first case, Google can deliver targeted ads right away, in the second it will have to take the context of the page into consideration. Since there are no other pages on the site yet, there is also no context and hence likely more fun with PSA or badly targeted ads.

Last question: How to remedy that situation? The answer is surprisingly simple: Just add more content to your website. A dozen of keyword strong pages work wonders. Also remember, that the banners do not change, till Google reevaluates your site. Reevaluation is triggered by people viewing pages, containing banner code, so you also need at least a few visitors.