Force flower help

I've got a question about force flowering. I live up in Canada and am growing white widow. I will need to force flower them while they are outside by covering them, but my question is how many days will i need to cover them for 12/12 so that when I quit covering them they wont go back to veg.

Thanks

Well if you force flower them you can't just go from 12/12 in the middle of flowering it could give you a hermie and screw up the prosses of flowering. It would depend on how long your plant buds and when you harvest it.