Minimum daylight hours?

14.5 is the actually trigger point of most strains.. At 15 hours of light they'll veg at 14 they'll flower.. That's working with just an indoor on-off lighting regime and doesn't translate well to moving vegged plants to outside..
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Just when depend on your latitude and the last frost dates for your zone.. The safe dates are May 1st to plant seeds outside as they'll need a month to mature and by then the days are more then long enough at all Northern latitudes to safely move a longer vegged plant out..
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Once a plant matures or in the case of clones they'll flower at the drop of a hat and putting out to soon results in early flower then reveg mid July and a really screwed up plant..
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The seeds I had sprout last week will stay under a 16-8 or a 17-7 light schedule till about June 1st.. I'm down at 33 latitude and don't have the much longer days sooner then you get further north so you can adjust your setout days slightly the further north you are..

BNW

 

Just leave them under that and you should be ok if you wait till late May - June 1st ish.. I've tried to gradually lower the light cycle but I was working with 24-0 plants and had nothing but flowering issues when I tried to introduce some dark..
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Time changes per day is like a wave cycle but averages out to around 30 seconds per day at our lower latitudes.. Somebody toss in a better number if they know one.. 3 minutes at say Seattle's latitudes
Since you already have established your plants on a dark cycle you can alter it but I'd adjust no faster then nature does.. Say 5 minutes per week..

BNW

 

Brassnorwood is spot on. At 31 degrees and almost exactly 14 hours I've got male flowers and pistils as of July 18.