We recommend 20 minutes after your last alcoholic beverage to be safe. Shorter times than this can mean that alcohol is sitting in the stomach and has yet to be absorbed into the blood. It may also mean that traces of alcohol are still present in the mouth. Either of these situations will mean that the breathalyser result will be higher than your actual BAC.
An easy way to tell if your BAC is rising or falling is to test yourself at 5 minute intervals. Once falling it will not rise again until additional alcohol is consumed.