depends how big you want your plants. I would say decide a height you want them instead of a time. Say you want them 2 foot in veg, then grow to 2 foot, then flower. Figure out how much space you have and use it to your advantage. I'd say measure your heigth to a certain amount below your light so you don't burn your plants. I don't know how far those lights need to be though, I'm not an expert in that.