Southern African varieties often flower with as little as 8 or 9 hours of darkness/15 to 16 hours of light. Other 3oth latitude varieties including most indicas flower when the darkness cycle lasts a minimum of 9 to 10 hours. Jamaican and some Southeast Asian varieties will trigger at 11 hours of darkness and ripen during September or October. Equatorial varieties trigger at 12 hours or more of darkness.

Some interesting stuff here:
Marijuana Growers Handbook