Before which engine speed do you have to downshift to engine brake effectively?

In order to engine brake effectively release the accelerator at 1,500-1,600 rpm and downshift when the engine speed drops to 1,200-1,300 rpm. Repeat this process as long as it is needed and is possible.

Using engine braking, you can slow the car without supplying fuel to the engine. Engine braking is much more energy efficient than using the pedal brake.