The amount on your monthly water bill will vary depending on your family size and water usage. 

The amount of water you use in a month changes depending on how many showers are taken,

 how much laundry is done, if the yard is being watered, and so on.

You will also notice a change in your bill if you have company for a long week-end,

or your family visits for a period of time.  This change is due to more showers, laundry, and general water usage.