Sweat loss is the weight you lost during the run plus everything you drank during it. Divide that by the time on your feet and you have a sweat rate per hour. One millilitre of sweat weighs about one gram, so a 1 kg drop plus 500 ml drunk over 60 minutes is a sweat rate of about 1.5 litres per hour. Weigh yourself with as little clothing as possible, towel dry after the run, and empty your bladder first for the cleanest number.
Sweat rates vary enormously between runners and between days, commonly landing somewhere between 0.5 and 2.0 litres per hour, and climbing further in heat and humidity. There is no target to hit. The value of the test is knowing your own number, in the conditions you plan to race in, so measure again on a hot day rather than assuming one cool-weather test covers race day.
The American College of Sports Medicine's position stand on exercise and fluid replacement recommends limiting body mass loss to under 2% during exercise. This calculator flags where your test session landed against that 2% mark. Losing more than that is common in hot marathons and is one reason finish times drift in heat, alongside the direct pace cost of the temperature itself.
Not usually, and few runners can. Matching sweat rate litre for litre is difficult at marathon effort and drinking beyond thirst carries its own risk, including hyponatraemia from over-drinking. Treat your sweat rate as the ceiling that explains how quickly a deficit builds, then plan a realistic intake that keeps you inside the 2% body mass guideline over the hours you will be running.
Substantially. Heat, humidity, sun and wind all shift how much you sweat at the same pace, which is why one test in cool conditions understates a hot race day. Run the test twice if you can, once in conditions like your goal race, and check what the temperature itself does to your pace in the heat pace calculator.