This will vary from strain to strain but most will start to flower when you drop below 14-14.5 hours per day. The shorter the flower time, the easier the strain will trigger.

Hey - so I have never tried the interrupted light cycle this early in the season, I am not sure how it would work. This early in the year supplimentary lighting might work better, but that's just sort of my gut feeling on it. The idea is to make sure your plant's don't drop below a total of 14.5 (give or take) hours of light per day.