Our typical induction is 6mg/kd total dose. We have done repeat induction with a similar dose (total 12mg/kg). I also know of centers who have treated rejection with similar doses (up to 7 infusions). I believe that you can watch the blood counts to determine the risks and utility of repeat dosing. I am not aware of an absolute limit.
Just recently SRTR has started collecting the the number of doses (not the actual numer of mg/kg), so I think we may have a better idea in the future.
My bet is it is a continuous variable, like pack-yr in smoking, where you have higher index of suspicion for cancer, etc, as the lifetime dose increases.
i think there is no definite answer. It is important to know if you are talking about rATG or eATG..meaning of rabbit or equine origin as vials are 25mg and 250 mg respectively