It will stretch according to the intensity of the light it's givin. and the strain it is, it looks indica so it probably won't stretch a whole lot. maybe a third to half of it's height.
I personally would wait till it gets about 24'' then you'll get bigger buds with more ability to gain as much yield as possible( maybe 4-5 if you give it what it needs with great plant care, being that you have just one plant, I'd think you'd want as much herb as possible?
I'd cut off the bottom few branches that will not get much light, use them as clones for your next round, wait for 24'' then flip to 12/12
General hydroponics Flora Nova (veg and bloom)series is an easy one part formula that I've used to success, maybe give it some cool bloom as an additive, and your good to go.