Now, I am quoting from a book. Best time to flower is when the plant shows you its ready by moving from veg to pre-flower under 24/0 or (better) 18/6. In the beginning, the plant developes nodes, with branches appearing in the middle space. After a while, no new branches appear but pre-flowers - experienced growers are even able to tell the sex in pre-flower. Then you switch to 12/12.

Of course, if you don't have the space... Hm. As far as I recall, mostly indicas will double their size during flower. Sativas are impossible to grow in a cab without pruning and training, which costs even more time.

I just completely burned one of my mostly indicas (long story, don't ask - just one of those days). They are now 3 1/2 weeks from seed, 2 weeks under a 400W HPS - last time I use that for veg, by the way. They are growing fine, but too much heat plus immense electricity is just not worth the trouble. They now have 6 nodes and develop their side branches - where additional bud will be placed eventually. My girls will get another 2-3 weeks of veg, probably, and then be sent to flower. I rather invest another week if necessary and get better results... but that's just me.