I reread the OP, and reconsidered the question, how many CAN I do before it hurts, and I am not sure I ever found out. Might be because of time and how I do it. In the past I would gear up and load a bunch so the machine would be available for other tasks. Now that I have two 650s that might change, but in the past I might just do a few hundred every time I had the time until whatever quantity was loaded. Now I will probably do 1k at a time for 9mm, so it is more likely that I will just kick that out in one session or two max.
If we are talking 9mm I would say that if you are not standing you should try standing.