Depends on the are you have to fill with it.. .or intend to fill with it...
when it is roughyl half LxW area, start bloom schedule. In future you can adjust this through trial and error, but that will get you in the ballpark.
most people grow them too big the first time...
they have ~3 weeks of vege growth left in them after 12-12 begins. it's deceiving how much more they grow.