i went out last year on the 21st of March with clones that had been vegged under 16 hours. The plant didn't flower until the end of july. It was an indica dominant strain. I put a bubba kush out at the same time, it flowered immediately and then entered reveg and then flowered again and never finished. It was on the north wall and received light from sunrise to sunset. my other plants grew on the west wall and received light from sunrise until 3pm as the wall shaded them. They were under ambient light from dusk until dawn. my west wall is great for summer even though it never gets more than 9 hours of direct sunlight. Any plant i put on the east wall flowers immediately and finishes there as well. It doesn't begin to get light until the afternoon any time of the year and has ambient light from sunrise to sunset. im in southern california by the beaches. what i think determines it is the changing of sunrise and sunset on a daily basis. i chart this and only go outside once i know that the days are getting longer. if you look up sunrise and sunset for your location you'll see that for a period of time the sunsets later each day but the sunrise doesn't move. the days may be getting longer but not by enough time i think. once the sunrise starts getting earlier and the sunsets are getting later the amount of daylight increases by a more drastic amount keeping the plant in veg. in the late summer the sunset begins to occur sooner each day and by the fall, the sunrise is occring later as well.
hope i wasn't rambling too much...