In baseball, if you have a .300 batting avg, your considered "good" or atleast semi-sonsistant. Whats a good CTR ?
It really depends on the sort of ad your displaying and the type of site you have, however, a value of 5%+ would keep most people happy.
I like 12-20%. I think that is your best bet. However depending on the traffic and nature of your site, you may get super low CTR if you have alot of traffic. My CTR drops to 2% when I get links posted on high-traffic humour portal sites.
I guess this would depend on way too many variables such as the topic of site, the ad placement, the relevancy of the ads, to name a few.
I've seen CTR go from basically nothing to sometimes 10-20% for a day. They can be all over the map depending on the day and I doubt if there is really a fixed ratio.