During your training you should know when your body needs nutrition and liquids.
Especially for longer workouts (more than 1.5 hours) you should not forget to eat and drink. Also during a race it’s essential to feed your body with new energy at the right moment.
How much should I drink?
It depends on many factors (intensity, temperature, etc.). If you do triathlon, don’t forget that you should drink more on your bike because it’s easier than during running. In general you should drink as much as you lose.
The scientific way
If you have a digital scale, measure your body weight before your workout and then shortly after your workout. Then take the difference of these two numbers. You have to do that in various situations, so you get your liquids consumption. If the weather is hotter, you need more liquids – easy or? 🙂
You also need nutrient and salt in your liquids. Pure water is good, mixed with apple juice is great. I always take natural salt in my liquids with me.
So, always stay hydrated.