2,098. Regular Self-Inspection
Hilchos Issurei Biah 8:11
It is praiseworthy for a woman to check herself frequently even if her period is expected at a fixed time because bleeding could always come at other times. During her eleven zivah days, however, there is a presumption that she’s clean so she need not check herself. After her zivah days, she should resume her self-inspections.
Hilchos Issurei Biah 8:12
If a woman neglects to check herself, either because of extenuating circumstances or by choice, she’s assumed to be clean until she checks herself and finds otherwise.