How many hours of light do I need for blooming?

12 hours of light and 12 hours of darkness.
12/12 is the standard cycle for flowering Cannabis plants indoors.

• If you extend the light period by an hour or two (13/11 to 14/10) you can correspondingly extend the flowering period by 10 to 15%. There is an added bulk that makes up for the extended time but the buds may be a bit leafier.
• Any attempts to use a cycle that does not add up to 24 hours should be considered very experimental as it goes against the natural rhythm that all living things have evolved with.
• Some equatorial based varieties may not be light cycle triggered at all. This means that they are used to growing under 12/12 all the time and will flower (and finish) when they are good and ready.