Residual chlorine of water should be:

Principles of chlorination:  The "chlorine demand" of the water should be estimated. The chlorine demand of the water is the difference between the amount of chlorine added to the water and the amount of residual chlorine remaining at the end of a specific period of contact at a given temperature and pH of the water. The minimum recommended concentration of free chlorine is 0.5 mg/liter for one hour. The free residual chlorine provides a margin of safety against subsequent microbial contamination which may occur during storage and distribution. Essentials of preventive and community dentistry  Soben Peter  5th edition