I'm sure it depends on where the bulk of the ads are shown (geographically), but in my case about 1800 clicks will earn $500. My CTR averages about 1.3%, so I need about 140,000 impressions to get those 1800 clicks. Keep in mind though, that this is across all my apps - so it's a mixture of banners & interstitials. Banners generally have a much lower CTR & a much lower Impression RPM than interstitials, but a much higher number of impressions. Personally, I think banners are pretty much a waste of time if you're trying to generate revenue because they are easy to ignore & compared to interstitials are boring - hence the much lower CTR. Ultimately it's a numbers game. The more impressions you can generate, the more you will earn - however there are ways to maximize that revenue (like using interstitials instead of (or as well as) banners.
There is always the concern about pissing users off with too much or too annoying advertising, but if publishing apps is your only source of income then you have to find the optimum formula for displaying ads & generating revenue - & realize that there will be people who won't use your apps because of those ads.
It always irritates me a little when I get feedback or emails complaining about ads in my apps, because those people are obviously happy to use my apps but they put zero value on the time & effort I put into developing them. For those apps that I have paid ad-free versions of, I simply reply by saying that if the ads annoy them too much, they can download the paid version. I have only ever had one person reply & say that they don't pay for apps - so I guess that person either puts up with the ads, or uninstalls the app.
- Colin.