A defrost should run just long enough to clear the evaporator of frost, then stop. Ending it too early leaves ice that builds up cycle after cycle; ending it too late wastes energy and warms the cabinet. Controllers therefore use an evaporator probe and stop the defrost when the coil reaches a termination temperature a few degrees above freezing.
A maximum defrost time acts as a safety limit. If the evaporator probe fails or is badly placed, the controller ends defrost after this time instead of heating indefinitely. On simpler units, termination is handled by a bimetal thermostat clipped to the coil, or by a mechanical timer alone.
When defrost termination goes wrong, the symptoms are distinctive. A coil that is iced in one corner after every defrost often means the probe is in the wrong spot or the termination temperature is too low. A cabinet that warms sharply during each defrost may have termination set too high or a probe that has slipped off the coil. Both are technician adjustments.