There isn't an accross the board standard as such but there may be norms in particular markets/industries. It does largely depend on the industry and the ad itself.
for best creative with standard size like 728x90 = 2 per 1000 views 300x250 = 3-4 per 1000 views 468x60 = 1-2 per 1000 views thats when creatives are good and working, for low grade creatve it can even be worst... also depending upon the traffic type.. the above stats are mostly for RON traffic and not category specific traffic!
there is not a standard...it totally depends on what the topic of the site is as well as a large plethora of other things...