As a general rule, you should drink any time you feel thirsty. Also depending on how intense your workout is and how much you are sweating will also affect how much water you need to b drinking. If you experience severe muscle cramps, dizziness, vision changes, get a headache or your heat is pounding you need to stop, sit down, and hydrate right away. Also be mindful of your sweat- if you stop sweating while working out it is a warning sign you are severely dehydrated!