Is it true that it takes 21 days before symptoms appear and you know you have scabies?

Scabies: No - generally once you come in contact with scabies you start to have symptoms itching. Head lice - usually can be seen by the "nits" attached to the hair shaft. Body lice usually affect the interdigit areas (between fingers) and cause intense itching. Pubic lice - usually causes intense itching in the pubic hair areas.

Scabies: Sensitization to scabies begins 2-4 weeks after the onset of infection. During this time the parasites may be on the skin and may burrow into it without causing pruritus or discomfort. Severe itching begins with sensitization of the host.(andrew's diseases of the skin clinical dermatology).