How can i tell how long my child has had head lice, by the number if nits?

Not by the number...: ...But by their location on the hair shafts. A head louse will attach her eggs (which is what nits are) to hair shafts at the point where they come out of the skin. As the hair grows, the nits move up from the scalp surface. Hair grows about 1 inch per month, on average - so if the nits are a half-inch away from the scalp, lice have been present for about 2 weeks; an inch away, 4 weeks; and so on.