Thirteen. Mine used to be every 26 days, actually. It finally dawned on me that six days before it hit was not a good time to test me, and I often failed. The native Americans' tent in the woods is a good idea. Imagine a woman's prison. When women are together in a group, periods sync up. Happened time and time again in shows - I did a lot of large cast musicals and this was the case, especially on tour.