How long depends on the strain, mine take 3 or 4 weeks to get a foot or so. You want a good root system going. I would top the hell out of it ( super cropping ) till it's about 1 1/2 to 2 foot, Then flower. It's going to keep growing at first 12/12. But don't top any more then. Maybe 3 to 4 foot when done. Or you can scrog ( screen of green ). I have not tried that yet but i hear and see the results are very good.