i decided based on space... i knew that if my plant could triple in height during 12/12, that i had better change the cycle when my plants got to 18 inches so i wouldn't risk getting taller than 4.5 feet.
i've read somewhere that it is ideal to grow the plant until it starts showing alternating nodes, because that's the plant showing that it's fully developed and ready to flower.
mine did start to show some alternating nodes by that height, which makes me feel pretty good about things...