Hair does not grow continuously. Each follicle runs through a cycle of growing, resting and shedding, and at any given moment the hairs on any part of your body are at different points in that cycle.
This is the single most important idea for understanding electrolysis results, because the method works best when a hair is caught in its active growth phase.
The three phases
Anagen (growth)
The active phase, when the hair is firmly attached and connected to the cells that feed it. This is the ideal moment to treat a follicle, because the target cells are present and reachable.
Catagen (transition)
A short phase in which the hair detaches from its blood supply and begins to move up, no longer actively growing.
Telogen (rest and shed)
The resting phase, after which the old hair sheds and a new one eventually begins to grow, starting the cycle again.
Why this means a course of sessions
Because only a proportion of your hairs are in the active growth phase at any one time, a single session can only treat the hairs that are currently in anagen. As the cycle turns and resting follicles become active again, those hairs are treated at later appointments.
Working with the cycle in this way, over a planned series of sessions, is how electrolysis achieves genuinely permanent results. Trying to rush it does not help, because follicles that are dormant simply are not ready to be treated effectively yet.
What affects your personal timeline
The size and density of the area being treated.
Whether hair growth is being driven by hormones, which can keep prompting new follicles.
Your individual cycle, since phase length varies from person to person and area to area.