How soon after coming in contact with someone lice does your head start itching?

Itching is not an immediate symptom of lice infestation. It typically takes about 4-6 weeks after the initial infestation for itching to start. This is because it takes time for the lice to lay eggs (nits) and for the nits to hatch into nymphs and adults. The itching is caused by an allergic reaction to the lice saliva, which is injected into the scalp when the lice feed.