The timing is not an exact science they will all likely finish at different times and since they are autos light switching may have little effect. Matter of fact I know a couple growers that ran 18/6 all the way through the grow with autos
Hope this helps
12/12 may actually jump start the flowering in a auto flower
You can leave them ynder 18/6 entire grow if you want
I actually put mine under 12-12 after two weeks under 18/6