I think it takes a long time, like up to two week to start experiencing hallucinations, but the brain differs from person to person, so there is no real way to tell at what point a person might start experiencing hallucination. While sleep deprivation up to a week can cause things like lose of memory and lack of concentrations are common, it usually take longer to start actually hallucinating. The brains different for everyone though, if you think you are hallucinating I'd get your ass to a doctor.