Seem to recall that the stay would hold the both windows open on a C152 up to about 60kts. Can't remember the exact speed limit as the only time I've done it was when my instructor was teaching me slow flying many moons ago.
A friend of mine used to do aerial photography for a living in a C150 and he used to detach the window stay and then let the airflow hold the window against the underside of the wing at a more comfortable 80 to 85 kts.