As a domestic disciplinarian, I truly believe that "enough is enough" when the spankee demonstrates that they have experienced a painful consequence and then express remorse or contrition. If sassy mouth attitude continues, or acting as if the spanking is not affecting them, or they refuse to hold position...then obviously they have not been spanked enough! I will continue to spank until they either 1) cry profusely and apologize or 2) apologize.
That is when I know that they have had enough.
Cheers!
Veronica D.