Most experienced MCers round these here parts use 1/4 tsp per drink or more.
FYI not all cayenne powders are created equal. They are classified by Scolville heat units or HUs. Most regular store bought cayenne is in the 20K to 40K HU range. According to 2 of the best known proponents of the healing properties of cayenne Dr Christopher and Dr Schulz cayenne needs to be in the 90K range or above to have theraputic properties so when I mention 1/4 tsp I'm taking bout the 90K or above.