Well, my experience shows that < 1.5% is bad. Aim for atleast 2% or possibly more than 3%. i never saw more than 4% but if it's possible, why not. Just don't do anything that might get you kicked out of program. What I would say, healthy CTR is somewhat between 1.5-3%.
To the replies who had low (sub-1%) CTR: if your keyword is competitive there are only two ways to get CTR up. One is optimizing your ad. The other is paying up.