Tractate Niddah - Chapter 9 - Mishnah 10 נידה פרק ט׳ משנה י׳

If a woman regularly observed [menstrual blood] on the fifteenth of the month, and then she changed and observed it on the twentieth both become prohibited. If she changed twice and observed it on the twentieth both [are still] prohibited. If she changed it three times to observe it on the twentieth then the fifteenth becomes permitted and the twentieth is set [as her regular day], for a woman does not establish a regular cycle until she sets it three times, and she doesn't lose her regular cycle until she is free from it three times.