How many weeks minimum is necessary to force Gorilla Glue #4 photoperiod to change to flowering?

You should see signs of flowering about 2 weeks after switching your light to 12/12. She won't flower at 18/6 like autos, you will need to go to 12 hours of light and 12 hours of darkness for flowering to occur. Your diary says "Gorilla Glue auto", but you are asking about photoperiod plants, which have an entirely different flowering mechanism. If your GG is meant to be an auto, it should be staring to flower by now (week 6), so you might actually have a photoperiod plant and will need to go to 12/12 to get flowering on its way. If you go to week 7 in my diary "cannabis conundrums", I have posted pictures of female pre-flowers/primordia and this may help you to identify what to look for and be able to determine what is going on with your plant.

Are you asking if you can flip yet? You can run a 12/12 from seed if u wanted them to flower ASAP, so you can do a 12, if its not there yet it will flip when its ready to. Obviously you will get a smaller yield if you don't run a longer veg. So you can flip now buy u may wanna wait. I'm guessing you want it to flower because the other is.