How does a pilot know when to initiate a turn from the downwind leg?

A pilot knows to initiate a turn from the downwind leg primarily by following ATC instructions. Air Traffic Control plays a crucial role in managing the flow of air traffic in the vicinity of an airport. They provide guidance based on the overall traffic situation, which can include instructions for turning at specific points, such as the base leg when preparing to land.

While personal preference may influence a pilot's actions, it does not provide the necessary situational awareness that ATC instructions offer. Approaching the approach speed or looking at other aircraft can also be factors in some instances, but they do not serve as the definitive method for determining when to turn from the downwind leg. ATC instructions provide clarity and safety by standardizing procedures and ensuring that all pilots are aware of the traffic dynamics at that point in time.