Backflow preventers must be tested on a regular basis to ensure that they are functioning properly and preventing contaminated water from entering the public water supply. The frequency of testing depends on several factors, including the type of backflow preventer, the level of hazard present, and local regulations. Generally, backflow preventers should be tested at least once a year, but some may require more frequent testing, such as every six months or quarterly. It's important to check with local authorities and certified testers to determine the appropriate testing schedule for specific backflow preventers.