In optimal conditions, plants should be kept in their vegetative stage for approximately 60 days. This time period should give the plant the opportunity to maximize yield and acclimatize to growing conditions. Ideally, you want the plants to finish at approximately 24 to 36 inches tall, so you will induce flowering when plants are approximately 12 to 18 inches tall. So if your plants are least an ft tall go ahead and start the switch. Hopefully, I answered your question and remember Happy Growing and Free your mind one puff at a time.