Keeping to the interval set by the hair cycle decides the result.
Laser energy is absorbed by the dark pigment in the hair and at its root, turning into heat that damages the structure driving growth. It only works on hair that is growing now, so it cannot all be cleared at once and sessions repeat in step with the hair cycle. Facial hair is fine, so more sessions are needed.
It takes about eight to ten sessions at three to four week intervals. The face has a lot of fine, pale hair with little pigment for the laser to be absorbed by, so fewer are caught per pass and more sessions are needed. The growth cycle is also shorter, so intervals are set tighter than elsewhere.