This picture shows the three stages of the head lice life cycle, including the head lice egg or nit, nymph, and adult louse, as compared to the size of a penny for scale.
Lice Life Cycle
A mature or adult head louse can lay up to 10 eggs or nits each day.
These nits, or lice eggs, hatch in about 7 to 12 days. Baby lice or nymphs are about the size of a pinhead when they hatch, and quickly mature into adult lice in about a 9 to 12 days.
In just a few days, adult lice are ready mate, starting this lice life cycle all over again during their 3 to 4 week lifespan.
Of course, a proper lice treatment regimen can interrupt the lice life cycle and help you get rid of the lice on your child's hair.
Sources:
American Academy of Pediatrics Clinical Report. Head Lice. PEDIATRICS Vol. 126 No. 2 August 2010, pp. 392-403.
