I am pleased as punch if they run under 80 C, I shut them down if they run over 90 C (sustained).  Those are my rules of thumb, some people push it more than others though.  How fast you have to run your fan depends on your ambient, your airflow (either in case or open air), and the actual cooler on the GPU itself.  Each one is different and only you know how much noise you can take. hehe  I don't like to run the fan at 100% fulltime, crappier fans do die out faster that way, I prefer 80-85% or less when possible.