Knowing when is enough can be a tricky thing. It also depends on what you're trying to do with your site. If your goal is only to sell stuff on your site then maybe having ads cover 95% of your site is a good thing. But if pushing content is your thing then maybe 30% is better. There needs to be a fine ratio between content and ads. This ratio can even be seen in the keyword/content relationship. Search engines start to reject sites that have over a 20% relationship. And people start to reject sites that have too many ads at a certain point too. Although no one has come up with a survey or poll to figure that out yet, it doesn't taker a rocket scientist to know your sites not working. Maybe they did have a survey page, but it had too many ads, I would have to say that the ratio should be between 20% and 30% depending on your page.

You page should be easy to navigate through. Your content should be easy to find. Yes, there are tricks to make the ads look like your material in order to trick people into clicking on them, but remember if people can't find what they want then they might not come back. Would you rather trick someone one time for a click or have them return for many more clicks.

You can experiment with your ads. Try some pages with 45% ad ratio and some with 30%. See how each of these pages work. You can even try different placement. Some people believe that Content should be aligned Top:Left. Others Center Frame:Left Heavy. Bottom line is the content must be in plain sight.

We live in a world that is quick, multi facet and multi tasking. People would rather click then scroll. So anything belong the fold (the term used for anything below the page that is displayed in your browser) is not important. Hence build smaller pages.

Now with smaller pages the ad content ratio can really get out of hand. So you need to know when is enough.