They'll stay in flower, I have had an issue with this before also GOD elaborated on this ~10 pages back, we do not have a long enough day ever to put a plant back to veg generally over 16hrs.
I may need to alter my stance on this subject a little.
I'm not sure that it is just the number of hours of daylight that matters - I think that if the number of hours/minutes of daylight is over 12
and the number is increasing each day, then the plant may believe that summer is approaching and revegging may happen.
The reason that I say this - I put a small flowering plant outdoors and the little fucker has revegged.
All I can say is that if light hours are constant (as in indoor growing) you need 16 hours+ to cause revegging. Outdoors, the number of hours daylight needs to be used together with whether the period of light is increasing or decreasing.
December 21st is generally the date when daylight hours start to decrease (in the southern hemisphere) so from mid-December onwards, a flowering plant should not reveg if placed outdoors.