Best hours to force flower?

Howdy all Im hoping you can help me out!
I have my 4 easy sativas growing very healthy but 1 is very behind.
The summer here is horrible and not much progress is happening so I'm thinking of force flowering my runt of the litter by putting it in a shed overnight in total darkness (Just as an experiment, I wanna learn all I can about all subjects in growing) and I want to know the best amount of hours to start forcing this plant.
any help much appreciated

12/12 light cycle total dark is required for, and it should bud within 10 days.
good luck

Cheers man thats helped alot, I'm planning on putting this plant in the shed at 9pm til 9am. As i said its only an experiment cos this plant isnt doing as well as its 3 big sisters.