"Population-based studies have attempted to estimate the prevalence of hallucinatory experiences in adults and have found the lifetime prevalence to be 8-15%"

From the introduction of
http://bjp.rcpsych.org/cgi/content/full/180/2/174
Occurrence of hallucinatory experiences in a community sample and ethnic
variations
The British Journal of Psychiatry (2002) 180: 174-178
Johns, L. et al.

It references these two papers
Sidgewick, H., Johnson, A., Myers, F.W.H., et al (1894) Report of the
census of hallucinations. Proceedings of the Society for Psychical
Research, 26, 259-394.

Tien, A. Y. (1991) Distributions of hallucinations in the population.
Social Psychiatry and Psychiatric Epidemiology, 26, 287-292.