8 drops per gallon is for clear water with no chlorine in it. 16 drops for cloudy or suspect water. You'll probably need less if you're using chlorinated city water. This ratio is directly from the Clorox corporation.
The best way is just to get a chlorine test kit at a pool supply place. Any of us who will be using chlorine to treat water really should have several of these kits put away. Because ratios are notoriously unreliable. Parts per million is the only reliable way to judge when there is enough chlorine. You're looking to maintain about 3 PPM or so. When it drops to 1 PPM, it's a good time to retreat it. The time it takes to drop will depend on the heat and such. I generally have to treat mine twice in summer and none in the winter.
By the way, bleach has a notoriously short shelf life. Calcium Hypochlorite (pool shock) is a dry form of chlorine that lasts for years. With it you can make bleach for water treatment or household sanitation. There are a bunch of really good threads on it that you can find with the search feature if you want more information.