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.
Particularly active people sweat about 1.5-1.8 liters an hour, while a triathlete can produce nearly four liters of sweat in the same time. During the Ironman Hawaii, competitors perspire some 15 liters (about four gallons) throughout the combined marathon run, 2.4-mile swim and 112-mile bike ride. Lawrence Spriet, an exercise physiologist at the University of Guelph in Ontario, says that after losing 3 to 5 percent of our body weight, the sweating process begins to slow down.
For the rest of the story: www.popsci.com/science/article/2013-07/how-much-can-human-body-sweat-it-runs-out