For good hydration during workout, drink 1/4 - 1/2 cup of water, every 15-20 minutes. Don't rely only on your thirst.

In warm weather you may need more because of increased water loss due to sweating. Your thirst mechanism is blunted during exercise, so don't rely on drinking only when you are feeling thirsty. Instead, set your watch to go off every 15-20 minutes to prompt you to take a sip.