I think the tolerance point is on a person by person basis. Depending on factors of condition of their digestive functions and perhaps some tolerance issues might have to do with the particular person blood pressure as cayenne is a circulatory stimulant. The MC was first developed to cure Ulcers so to be honest I don't really know if it would have any adverse effects on the stomach lining if you can take it when you have ulcers. The preceding statements are stricktly an educated guess.
BTW cayenne is an excellent anticeptic. I am an avid whitewater canoeist and do a good bit of wilderness tripping and always carry a bottle of cayenne in my first aid kit both for it's anticeptic properties for any wounds and also it would be good as a strong tea in case of a heart attack or to help someone get warmed up if they get hypothermic.