Properly maintaining water chemistry is a very important aspect of owning a swimming pool. The water quality in your pool can not only affect your health but it can also affect the swimming pool itself and the equipment which circulates the water. That being said you can begin to understand the importance of properly maintaining the water in your pool.
People often assume that by adding more chlorine to the pool they will effectively be keeping the pool water clean. They also tend to think that with more people in the pool they will need to add more chlorine. The thing about the water in a swimming pool is that you will never know what it needs without testing it. This is why test kits were made. Typically with a heavy bather load in the pool the water quality will decrease. This is due to the fact that the oils in our skin as well as the sun screen that we use get into the pool and bond with the free chlorine preventing it from effectively killing the bacteria and viruses that may be there.
This depends on a couple of things. Typically the water quality will not change much during time of inactivity. However, one thing that can change is the volume of water in the pool. If the volume changes, then the water quality can begin to change. For instance, if the pool is left open and experiences heavy rainfall throughout the time of inactivity then the volume will increase. It may not be much, but it may be enough to dilute the amount of chemicals in the pool. In this case you would need to add more to stay within the appropriate range necessary to maintain a clean, healthy pool. Conversely, if over the time of inactivity in the pool, the pool is left open and experiences several sunny, windy days then you may find that the water volume in the pool has decreased. This is due to evaporation caused by the sun and wind. In this case the amount of chemicals in the pool would seem to increase. In both cases the amount of chemicals in the pool has remained constant, but because the volume of water changed it will either dilute or concentrate the chemicals in the pool.
The amount of activity your pool receives weather it is more or less will affect the water quality to some degree. All in all it is really best to test the pool water at least once per week to get a good idea of what it needs in order to maintain a clean healthy pool.