The Nazis actually had a policy of killing their slave doctors every 6 months. There was no, "until they stop being helpful". It was mandated to be 6 months. (but some camps were slightly more lax about it)
It was all pretty horrific, and the worst part is, the Nazis themselves never had very competent doctors, which is why they forced actually competent Jewish doctors to carry out the twisted experiments.
There are very few books written by people who survived the camps, and the one by Miklos Nyiszli is likely the best account of what these doctors were subject to.