I don't think you can OVER do it, but you can do it wrong. When you're trying to desensitize a spot, if you stop too soon, you actually end up SENSITIZING it. There's a good video on the subject called "Early Learning" and a book titled "Imprint Training". Both of them are by Dr. Robert Miller D.V.M. I've got the video and found it very helpful. I ordered them from Jeffers, but I'll bet you can get them a lot of places.
I just turned the horses out, and Cyn, still in heat, immediately backed right into Bay, who is no longer very interested, so he just took the opportunity to scratch and rub his face on her butt.
The look on HER face was hilarious, and I wished I had the camera. Disappointed, frustrated, and I think a little ~insulted~ at the lack of interest.