The US EPA protocols require that all short-term tests be conducted for a minimum of 48 hours. During the tests, all windows and doors should be kept closed and ventilation and exhaust fans should be turned off. Ideally, however, you should do one year-long test or several short-term tests throughout the year to determine the annual average radon concentration.
The US EPA has set an “action level” of 4 pCi/L (picocuries per liter). Levels equal to or above that amount carry enough risk of lung cancer from exposure to radon to warrant action.