The Ohio Department of Health recommends retesting a single-family home every two years and after any significant renovation. Test sooner if you start using a lower level, replace the radon fan, or change the sump, waterproofing, or HVAC.
More detail
ODH sets a longer clock for bigger buildings: multifamily, school, and commercial buildings should be retested every five years or after significant renovation. In a house with a mitigation system, the first test after the install belongs inside Ohio's window of 24 hours to 30 days after the fan starts, and the installer is required to advise you in writing to retest at least every two years after that. Between tests, the gauge on the pipe is the everyday check. A two-year rhythm fits the climate zone 5A heating season well: pick a month between November and March, close the house the way the testing protocol asks, and keep each result with the install paperwork so you can compare year over year. The retest schedule lays out the full timeline.