פירוש על נדה 3:3

"Every woman that has a regular time (i.e. menstrual cycle)" - It's set three times [in a row] (i.e. three regular menstrual cycles, one after the other), and she checks herself (i.e. performs a bedikah) at her time and finds herself ritually impure.

"...her hour (i.e. the time at which they see menstrual blood) is [a] sufficient (starting point for determining whether the terumot and taharot they handled are ritually impure)" - And [one] cannot say that it (i.e. the blood) was there orginally, for surely a guest [i.e. menstruation] comes on time.