How accurate are polygraph tests? There have been several reviews of polygraph accuracy. They suggest that polygraphs are accurate between 80% and 90% of the time. This means polygraphs are far from foolproof, but better than the average person's ability to spot lies. How accurate are polygraph tests? There have been several reviews of polygraph accuracy. They suggest that polygraphs are accurate between 80% and 90% of the time. This means polygraphs are far from foolproof, but better than the average person's ability to spot lies. How long are polygraph tests? While the length of polygraph examinations varies, the minimum polygraph examination time is approximately two (2) hours.