How tall should the plant be when I switch the light cycle?

I'm going 5 AMS(anti-mould strain) plants, they're at about 4.5 inches tall at the moment and I want them to finish off at about 3 feet tall, so when should I switch the cycle so that they grow to be 3 feet?

Thanks

Crystalcrazy said:

so when should I switch the cycle so that they grow to be 3 feet?

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about 12 inches

Yeah anywhere between 12-14 inches. If I were you and 3 feet is your max then I would go with 12. Each strain will act different when you switch to 12/12 some may tripple in size and some may only stretch 6 inches. So if this is your first time with this strain maybe google how much it stretches in the first two weeks of flower.