Dietitian Kristen Smith stated that she adheres to the 12-hour rule for a glass of water. After 12 hours, she chooses to pour and drink a new glass of water. However, she claims that you can drink a day-old glass of water as long as it hasn't been exposed to contaminants or left uncovered. "During the first 12 hours, there won't be enough nutrients for the bacteria count to increase. After 12 hours, there will be food for bacteria to multiply," the dietitian advised. If you're drinking water from the previous evening, it's probably fine. If the water has been sitting all day, it's better to take a new glass of water even if you've used a water filter. Additionally, sharing water with someone else accelerates the timeframe when you should discard it, writes Huff Post. "After a person drinks directly from a bottle or glass, bacteria from the mouth can transfer to the remaining liquid and start to multiply. That's why it's best to drink the water all at once and discard it, rather than saving it for later. However, if the only option you have is that bottle of water that's been sitting all day, then it's better to drink it than to remain dehydrated," said Smith.