I want to know how Admob pays for banners? Is it based on time or impressions or count of times that been showed to users?
Admob, like most ad services, computes your earnings based on impressions (a user saw an ad) and/or clicks (a user clicked on the ad).
Admob uses an auction model, so the actual monetary value (to you) of a given impression or click isn't fixed and will depend on what the people buying the ads have bid for the space. Admob will attempt to display the type of ad that results in the most revenue for you.
You can view historical trends regarding your earnings in your control panel, along with a variety of reports and metrics that may help you estimate the future revenue for your ad campaigns.