The real question is, how much height do you have in your room?
I would let it grow until it was anywhere between 1 - 3 feet from the light at its highest point. I say this because most strains will grow an additional 1 - 3 feet after you flip into flower, and you want to make sure you have enough room to support that.
It's going to depend on your strain too, as well as if you top it or not, and if you use a product like bushmaster or phosphorload. If you are going to be growing sativa's or sativa dominant strains, you might want to give yourself 2 - 3 feet. If you are going to be growing indica, or indica dominant strains, you should give yourself 1 - 2 feet.
For the most part if you aren't using all of your height to grow in, you are losing money. Not in all cases, but many. Since you are paying to cool or heat the area, add or remove humidity, you should be using as much of your area as you can.