We've all heard the saying that adults should drink 8 cups of water a day, but that is mostly an urban legend based on guidelines given decades ago without much scientific substantiation. It's practically impossible to give fixed guidelines on drinking water because everyone's needs are different.
Think about it this way: would it make sense to recommend that a regular runner who lives in a dry climate drink the same amount of water as someone living a sedentary lifestyle in a humid location? How about for people that sweat fairly easily versus those that hardly ever do?
Not to mention your other sources of hydration, such as your morning cup of coffee (almost entirely water) or the fruits and vegetables you eat (75% to 95% water). On the flip side there are things that dehydrate you, like alcohol or certain medications.
Just as we recommended with regards to your diet, when it comes to hydration you should be your own advocate. Everyone's hydration requirement is different. Pay attention to how thirsty you feel, and also the color of your urine. If its a darker and a richer shade of yellow you probably need more water. If its almost clear you are probably in good shape.
How much you drink is just as important as how frequently you drink, especially during exercise. It's better to take small sips throughout your workout than waiting until you are thirsty and chugging. Since your body takes some time to deliver the water to the needed locations, its also a good idea to start hydrating before you start working out.
As for replacing important electrolytes, check out our post on drinking coconut water.