Reply,
My experience with double cone blenders is that they only need to operate for about 5 minutes to get the powders mixed properly. In fact, I have heard that if you mix for longer periods of time, the powders can, to a degree, become unmixed. Do not fall into the belief that the longer you mix it the better your chances are of getting the powders well-mixed. As for the RPM, large double cone blenders usually operate at a few RPM. I believe that the larger blenders tend to operate at slower RPMs than the smaller ones. I hope this helps.
Good Luck.