Take a look at this pretty interesting post on impressions and revenue with advertising in Windows Phone 7 apps.
Source: OccasionalGamer
A lot of people have asked me what the typical cycle is with ad revenue and there is fear that their earnings are just a temporary spike in ad revenue and that it might dry up pretty fast. I used to have the same fear, but lately I have been looking at data more to alleviate these fears, as most people would know, the fear of the unknown is big.