Since a flea can lay eggs for 12 days, it takes borax approximately 2 to 6 weeks to control the whole cycle of fleas. Borax is more effective in killing the eggs and larva than the adult fleas, making the time duration it takes to fully control the pests vary.
Borax is a boron compound also know as sodium borate, sodium tetraborate, or disodium tetraborate, which kills fleas by dehydrating them and their larva, as well as facilitating their eggs to hatch and dehydrate. After sprinkling the borax on surfaces infested by fleas, it remains potent long enough to kill any new fleas that may get onto the surface. To prevent any side effect to pets, make sure to vacuum any remaining particles of borax, dead fleas, larva and eggs after 24 to 36 hours.