It depends on how much taffic you are getting... I've heard of 75% on pages that only had a few page views on a particular day, but overall it goes down to a more realistic level.
I've heard of people get over 100% with really low traffic. Some of their users hit the back button and clicked on a different ad a couple times. My click-through is usually in the mid 20's and I get thousands of page views a day.
One of my sites has had 440 visitors this month and has 295 clicks, this site has a very high CTR indeed.
I've had 24%, (400 views 97 clicks), but I allways have from 10% to 14%, it shoud be a day with a very matching content ads.