I think it's different for every female. Some can carry a higher bodyfat better than others. Of course body fat measurements are so varied and often unreliable, it's hard to arrive at any one number that's "right". For general health, the upper limit for women is usually quoted as being in the low 20's. Athletes are usually in the lower teens. For competition, bodybuilders and fitness competitors go into the single digits, but don't stay that way year round.
As for bodyfat affecting menses, that's different for different women as well. I think cessation of menses is related more to stresses on the body (overexercise, undernourishment, etc.) than actual bodyfat percentage. A lot of people say periods stop below 12%, but that's not true for everyone.