16-20 hrs is pretty ideal unless you just want it very strong. Be sure to mix the grounds and water well (let the grounds 'bloom' with the water for a minute before sealing/covering). The grounds ideally will be on the course side and fairly fresh. 4-to-1 ratio of water to grounds is what I use. Medium to dark roast is good to get a strong flavor, and cold brewing doesn't extract much bitterness at all. In my personal experience more floral/fruity-forward beans/roasts don't work so well for my flavor preferences. I enjoy hot brew of fruity/floral beans, but as cold brew it's just not very appealing in my opinion.