The length of time an African violet needs light depends upon the intensity of the light. Assuming that you have bright, but not hot light, an African violet will do well with between 8 to 12 hours of light a day. To decide where, in the range between 8 and 12 hours, see the question "How can I tell if my plants are getting too much or too little light?" Most plants require an amount of darkness each day. African violets are no different, and should have at least 8 hours of darkness.