All induction cooktops that I've heard of or used have sensors to detect when there is enough ferrous pan material above them to work correctly. (I haven't tested my quasi-commercial Cooktek wok hob though.) This is a safety measure as well as a convenience for the design of the electronics. (The hob coil and circuitry is the actual sensor.) If you lift a pan off an active hob, the cooktop will notice within seconds and throw a code, then try to detect whether the pan has been replaced. During the brief time that the hob is powered and the pan has been removed, the hob power is only virtual, and not heating the air or glass. After the code is thrown, the hob is basically unpowered.

If the pan remains missing, then the time that the cooktop circuitry continues testing for a pan may vary with manufacturer. I just checked my replacement Frigidaire Gallery unit and the hob control continues flashing the power setting seemingly indefinitely (I gave up after 30 seconds). It may eventually stop. The hob remains safe. I vaguely recall my previous Kenmore (also made by Electrolux) shutting down after a while, but I won't be able to test that unless I repair the unit.

kas