Well, the blooms last for just a couple of weeks. You don't have to worry about the growing if the soil is rich enough or if you fertilize it often.
As far as i know, in marijuana, when the flowers are pollinated and the formation of seeds begin, the THC amount decrease.
But i am not sure if when your flowers are pollinated, the mescaline amount decrese or not. But it doesn´t matter since you are not going to cut off the cacti for eating it soon.