As recommended by the CDC, it’s important to have your well water tested once a year for total coliform bacteria, nitrates, total dissolved solids, and pH levels. That’s the minimum, though, as you can always test your well for additional contaminants and maximum safety.
You should also have your well tested if you're concerned that an environmental factor affected your well. These can include a problem with nearby wells, a recent flood, a change in your water quality, or a recent repair to your well.