Jaideemaak - 3:41 pm on Nov 9, 2011 (gmt 0)
Agree with londrum. It's based only on previous experience for my own sites. If one day the CTR is x and the next it is one-tenth of x I consider it a lousy day.
I might do this day by day, week by week or month by month. I've given up comparing year-on-year because that exercise is really depressing.
I'm not sure that your direct mail analogy is relevant. Yes, Adsense is easier for publishers but it's also easier for visitors.
Selling to people via direct mail requires them to part with some money so it is a difficult sell, and 1%-2% would be very good. Clicking on an ad costs nothing so there is no need to overcome the initial reluctance to buy. It's completely risk-free.