Easy weeks are done for recovery. They should be timed when recovery is needed and most beneficial based on the amount of fatigue one is carrying from the past several weeks.

For most people 3 and 1 works very well but in the absence of actual training data it is at best a guess.

A free subscription to Training Peaks will provide a certain degree of accuracy. TP uses a number called "Training Stress Balance" or TSB which indicates the balance between stress and fatigue. When it gets too low and stays there then it indicates that some recovery is needed.

-Tim-