The "standard" measurement is: 1 cup per 15 gals of water.
Now keep in mind that there are different levels of chlorine mixes. The typical house hold bleach (Purex, Clorox) is only at 3%...whereas pool chlorine is a lot more. The above measurement is for house hold bleach.
FWIW, your owner's manual will probably have a "winterization" section that will stipulate how much to use. Check it out first.
Ron