Forceing early flowering

So I have been LSTing my plants(Barny's Farm LSD)since they had 4 sets of leaves. Love the results, will never top again. Anyways, so I picked up some 50 gallon rubbermaid containers and painted them with 3 coats of flat black paint and started covering them up today. I'm doing 12/12 light treatments. Does anyone know how long I will have to continue the treatments for, before they actually will continue flowering without containers being placed over them. 1 or 2 weeks of covering them for 12/12 or longer? Any help would be greatly appreciated. There are still about 15 hours of day length here. I'm during this cause I had to pick last year's crop early cause it turned bitterly cold towards the end of september and the plants had just not finished in time even know they where 7 and 8 week flowering strains. This year I wanna make sure they finish.

A good 2-3 weeks easy once they start to flower they'll keep flowering along as the light does not go over 16 hrs

are you sure the trash cans are dark enough? maybe throw a tarp over em for some extra darkness. if you have an extra can, stand in it and put the black one over you. see what happens.

i use this for checking daylight hours http://www.timeanddate.com/worldclock/sunrise.html