Factors that determine when and how often your water should be tested would involve

Source of water: If there is any change in the source of water then it is advisable to go for testing. Any repeated patterns of sickness in the family or any indication of water contamination, then testing becomes essential.

The ideal solution would be to test once a year and as and when any changes have been made in the source. Remember, one day after testing and finding "no contaminants", your source could become contaminated.