It is estimated that the average person loses about 2.5 liters or 84 ounces of fluid per day whereas athletes tend to lose even more with exercise. Fluid intake is important for metabolic, thermal regulation and waste removal purposes to allow for normal body functioning. Factors that go into hydration and sweat levels include temperature and humidity, clothing material and layers, hydration and nutrient levels prior to exercise, and fluid replenishment during and after training.
How much should you drink in order to replenish the water your body has lost through sweat and evaporation? Heres how you find out. Before you exercise step on a scale wearing as little clothing as possible (clothes can become drenched in sweat in some cases or if someone is just a heavy sweater). Go through your workout monitoring how much fluid you drank and then post workout re-weigh yourself. Now your body has not had time to drop pounds and pounds of body fat in a normal workout time period so what actually lost was the water you should aim to consume on top of normal hydrating periods prior to your next workout.
If an athlete weighs 210.5 and then exercises for an hour after which they measure that they have consumed 14 ounces of fluid and then re-weighs in at 209 it can be calculated that they had a sweat rate of about 38 ounces per hour (1.5 pounds =24 ounces + 14 ounces of fluids consumed). If this person normally drinks 84.5 ounces lost according to the average, in order to achieve full replenishment they would add on 38 ounces. Now your sweat rate and intake is probably going to look a little different than this so be sure to assess yourself on multiple occasions to establish a baseline.
The longer you train the more fluids you'll need to drink to recuperate and return to euhydration.
Especially as the weather gets warmer you'll find that even with very little exertion you tend to sweat in the sun or warm environments. The goal of hydration for performance purposes is to avoid a large deficit in fluids or electrolyte balance. A large amount would qualify as >2% of your body weight. Eating and drinking a few hours prior to exercise is recommended to allow time for absorption and a return to normal levels. Another strategy is timing your workouts to avoid the peak heat of the day to train in the morning or evenings when possible as the temperature and humidity increase. Even with temperatures in the mid 80's, relative humidity factors in to make it feel hotter and with a higher relative humidity the body will not be able to cool off as efficiently since evaporation of sweat happens at a slower rate.
Whether you want to get in your shrugs and lug a gallon of water around everywhere all day that's up to you. Another option is to take notice of urine color and frequency. One thing to keep in mind with that is to moderate and lower your fluid intake closer to bed time. This will prevent you from having to get up to make several trips to the bathroom in the middle of the night which inherently effects sleep quality.