Well that depends on how much room you have to grow. Are you growing in a box? If I was using a 4 foot grow box I would start to flower at about 15 to 18 inch's tall. When you flower the plant will double it's size and lots faster then it did when you Veg it.You can start to flower even when the plant is 8 inch if you wanted to depends on the space you have....Happy Harvest.....