How much is too much violence in thrillers?
This is completely subjective, and will vary from author to author. The use of violence is a tool. Some writers (and this is not just with thrillers writers) have to show every blow-by-blow, every punch or kick or knife strike. Then, there are others who show virtually no such specific action.
In order to do it right, violence must serve a purpose. But, do not underestimate the power of the reader's imagination. I recently came to grips with this on chapter 25 of my thriller Beholder's Eye. The chapter centers around a teenage girl, who is kidnapped by a serial killer. The chapter ends with her death. During the first draft, I showed her very violent, very bloody death. After thinking about it, I ended with the butcher knife being raised above the killer's head. It would serve no purpose to show her death because we actually see a similar death in chapter 2--I don't need to show it again.
Also, I liked the way the chapter ended, because it leaves the reader wanting more.
Read some of your favorite authors. How do they deal with violence?