Force flower question

So I have three plants, all about 3 months old, and I want to force flower them now. I just put black trashbags over them. How long do I leave them on for? Do I have to remove them and put them back on at twelve hour intervals?

hmmm.... Where do you have these plants? Usually one just switches the timers on their lights if indoors...

In not sure how to upload pics from my iPhone, but it's in a barn and it's up in the loft of the barn. The plants are underneath a large skylight, and I have two light fixtures with plant bulbs shining on them.

The skylight shouldn't be a problem because the days are short, but you should get a timer on the lights, set it to 12/12, and don't disturb them

Okay , I'll take the bags off tomorrow