It all depends on the size, physical fitness and hydration of the person in question, but it’s possible to sweat buckets before heatstroke sets in and we pass out. After all, there are about three million sweat glands on the human body (the highest concentration is on our palms), and the average person aggressively working out perspires about 0.7 to 1.5 liters per hour. Theoretically, if we were attached to a treadmill and pumped full of liquids, it’s possible to keep sweating forever.