The American College of Sports Medicine offers a guideline that says to drink 5 to 12 ounces of fluids every 15 to 20 minutes during a marathon.(21) If you are doing this with just water, you risk overhydration, so make sure you are taking in electrolytes as you go. More importantly, though, is that this is going to vary from athlete to athlete. To figure out just how much sweat you’re losing and how much you need to drink throughout 26.2 miles, try the "sweat test." To perform the sweat test, weigh yourself before and after your long run.