You can flower it now if you have time or space restrains. Personally I'd leave it at least get to 12-18 inches. The plant will between double and triple in size during flowering. It is really up to you, the longer you leave it in vegetation, the more budding sites it'll have and the more it will yield. To begin flowering all you need to do is switch the light cycle to 12/12 (that is 12 hour light followed by 12 hours UNINTERRUPTED darkness). Happy growing!
Try and keep the temp between 72-78F when the lights are on with a 5-10 degree drop at night. It is quite a forgiving plant though so don't worry if you're slightly out.