Unfortunately, we don’t have a Star Trek disintegrate ray to instantly destroy hair. The modern lasers we do have work very well, but takes several treatments. Multiple treatments are needed because of how hair grows.
At any given time about 30% of your hairs are in an active growth phase. The laser works by hitting the actual hairs, so if they are asleep the laser is ineffective. With a perfect treatment, only the active hairs, or 30%, can be removed at any one time.
When you reach 5 to 8 treatments you start getting close to complete removal. There is of course some variability from one patient to the next. In a perfect world 100% of the hairs would be destroyed with each treatment.
Unfortunately, the treatment is not 100% effective. It’s important to start at safe ranges of energy and slowly increase until optimal treatment is obtained.
This initial safety curve reduces the effectiveness of the initial treatments. Once optimal energy is obtained, differences in skin color and hair consistency can affect the treatment. On average most people see clearance around eight sessions. However, with darker skin and lighter hair each treatment is slightly less effective and may require more sessions.