A head lice infestation will not go away on its own and can last indefinitely if left untreated. Individual lice live about 30 days on the scalp, but they reproduce continuously, so a colony sustains itself for weeks, months, or even longer. With proper treatment, you can clear an infestation in roughly one to three weeks.
The Life Cycle on Your Scalp
Understanding the lice life cycle explains why infestations persist and why treatment timing matters so much. A single adult female lays eggs (called nits) that she cements to individual hair shafts close to the scalp. Those nits hatch in 6 to 9 days. The newly hatched nymphs then go through three molts over about 7 days before reaching adulthood. Once mature, an adult louse can live up to 30 days on a person’s head, laying more eggs the entire time.
That means from the moment an egg is laid to the end of that louse’s life, a single generation spans roughly 6 weeks. But generations overlap. By the time the first batch of nymphs matures, the original adults are still alive and laying new eggs. This is why an untreated infestation doesn’t burn itself out. It’s a self-replenishing cycle that only stops when you break it.
How Long Lice Survive Off the Head
Lice are poorly equipped to survive away from the scalp. An adult louse that falls off a person dies within 1 to 2 days without a blood meal. Nits that end up on pillowcases or furniture need the warmth of the scalp to develop, and they typically die within a week at room temperature.
This means the window of concern for your household is narrow. Any clothing, bedding, or fabric items used by the infested person in the two days before treatment should be washed in hot water (at least 130°F) and dried on high heat. Items you can’t wash can be sealed in a plastic bag for two weeks, which is more than enough time for any stray lice or nits to die.
How Long Treatment Takes
The timeline to clear an infestation depends on which treatment you use. The most common over-the-counter option, permethrin lotion, kills live lice but not unhatched eggs. It does continue killing newly hatched lice for several days, but you’ll typically need a second application around day nine to catch any nymphs that hatched after the first round. That puts your treatment window at about 9 to 10 days minimum.
Prescription options can shorten the process. Ivermectin lotion is effective for most people as a single application, with no nit combing required. Spinosad, another prescription treatment, kills both live lice and unhatched eggs, so retreatment is usually unnecessary. If live lice are still crawling seven days after the first application of spinosad, a second treatment may be needed, but that’s uncommon.
Regardless of which treatment you use, the confirmation point is the same. If you see no live, crawling lice three weeks after treatment, the infestation is gone. Any nits that were viable would have hatched by then, so three weeks of no activity is the reliable all-clear signal.
How to Tell If an Infestation Is Old or Active
Finding nits in your hair doesn’t always mean you have an active problem. Nits are glued to the hair shaft and stay there even after they’ve hatched or died. As hair grows, those empty shells move farther from the scalp. If the only nits you find are more than a quarter inch from the scalp and you don’t see any crawling nymphs or adults, the infestation is probably old and no longer needs treatment.
Active infestations have nits close to the scalp (within a quarter inch), and you’ll usually spot at least a few live lice, particularly behind the ears and at the nape of the neck. Itching is common but not universal, and some people carry lice for weeks before noticing any symptoms. This delay is one reason infestations sometimes seem to last a long time: by the time you feel itchy, the colony may already be well established.
School Policies and Missed Days
If your child has lice, they don’t need to be pulled out of school immediately. Both the American Academy of Pediatrics and the National Association of School Nurses recommend against “no-nit” policies, which require a child to be completely free of nits before returning to class. Their reasoning is straightforward: most nits found during school screenings are old casings unlikely to hatch, nits bonded to hair shafts don’t transfer to other people, and misdiagnosis by nonmedical staff is very common. The CDC’s guidance is that a child can finish the school day, get treated at home that evening, and return to class the next morning.
Why Infestations Sometimes Seem to Come Back
Recurring lice are one of the most frustrating experiences for families, and there are a few reasons it happens. The most common is incomplete treatment. If you use a product that doesn’t kill eggs and skip the second application, surviving nits hatch and restart the cycle within days. Resistance to permethrin has also increased in some areas, meaning the first-line over-the-counter product may not work as well as expected.
Reinfestation from close contacts is the other major cause. If a child’s best friend or sibling also has lice and isn’t treated at the same time, the two can pass lice back and forth indefinitely. Treating all affected household members and close contacts simultaneously is the most reliable way to break the cycle for good. Combined with thorough treatment and a check at the three-week mark, most families can resolve a lice infestation within a single month.