great responses..i was going to say 16.6(repeating)%..lolol
under 10 hrs of lite they would only hurry all the faster to the end. This brings us to critical day length.
The plant "senses" the longer nights by a direct interaction with light. A flowering hormone is present during all stages of growth. This hormone is sensitive to light and is rendered inactive by even low levels of light. When the dark periods are long enough, the hormones increase to a critical level that triggers the reproductive cycle. Vegetative growth ends and flowering.
The natural photoperiod changes with the passing of seasons. In the Northern Hemisphere, the length of daylight is longest on June 21. Day-length gradually decreases until it reaches its shortest duration on December 22. The duration of daylight then begins to increase until the cycle is completed the following June 21. Because the Earth is tilted on its axis to the sun, day-length also depends on position (or latitude) on Earth. As one moves closer to the equator, changes in the photoperiod are less drastic over the course of a year. At the equator (0 degrees altitude) day length lasts about 12.5 hours on June 21 and 11.5 hours on December 22. In Maine (about 45 degrees north), day-length varies between about 16 and nine hours. Near the Arctic Circe on June 21 there is no night. On December 22 the whole day is dark. The longer day-length toward the north prevents marijuana from flowering until later in the season. Over most of the northern half of the country, flowering is often so late that development cannot be completed before the onset of cold weather and heavy frosts.
The actual length of day largely depends on local conditions, such as cloud cover, altitude, and terrain. On a flat Midwest plain, the effective length of day is about 30 minutes longer than sunrise to sunset. In practical terms, it is little help to calculate the photoperiod, but it is important to realise how it affects the plants and how you can use it to your advantage.
Cannabis life cycle generally needs about two weeks of successive long nights before the first flowers appear. The photoperiod necessary for flowering will vary slight with (1) the variety, (2) the age of the plant, (3) its sex, and (4) growing conditions.