You can start flowering at any time, BUT the larger the plant is in veg, the larger it will get in flower and the larger your yield will be. Now I have flowered plants that were smaller than the one you have there, and i got a good 1/2 ounce off them, but don't expect that much your first go. Plus I use hydroponics, which is much faster growth. Expect your plant to double in height the first 3 weeks of flowering schedule.