Istishab cannot defeat an established legal proof

Argument Without New Proof: Absence of a Cause and Presumption of Continuity

والحيض لا يرتفع إلا بدليل
The legal status of menstruation is not treated as ended without proof.

This is not an unlimited principle. An established status requires evidence of change, but where a specific legal limit or other decisive evidence governs the duration, the presumption yields to that proof.

Istishab is used when change has not been established. It is not an independent competing authority that overrides direct scripture or other recognized evidence.