Since laser/ DPL/IPL/hair growth/hair removal works effectively for hair only in anagen, when to resume your next  session? Not too soon while more density of hair is still required & not too late when hair grow out too long. These can ruin your ultimate result. Resume next session as soon as majority of your hair start emerging out from skin. Good time to have treatment is when majority of hair is in anagen, only then u will get the most ultimate reduction result. The most effective result is when hairs are in the anagen phase. Why? Any dark-colour objects (in this case, dark skin & hair shafts of all growth cycle including anagen) absorb light from laser and turn the light into heat and give off such heat to surroundings that have no dark pigments,such as colourless follicles containing hair stem cells. Any colourless, red, blond, grey & white objects don't absorb light produced by the current DPL/IPL/hair growth/hair removal devices. Only hairs in anagen phase are still in contact with the parts of colourless hair follicles containing colourless stem cells. Even though hairs in other growth phases absorb light from laser and give off heat to surrounding but they have lost contact with stem cells. If the stem cells aren't destructed, hairs will eventually grow back. The subsequent IPL/laser hair removal session, the less the result.