For one niche its 3% for me and other is close to 1% and so niche is also important because at times when people of your niche knows this is advertisement they tend not to click
I think 5% is quite good.
I heard that it shouldn't be more than 15%, otherwise google will seriously check your blogs to see if they can find anything wrong.
I agree with Chrishirst - do people actually read the T & C's? if they did then they would know talking about your CTR is a big no, no and can get you banned from adsense in an instant!