How much you need is highly personal. Some people need relatively little. Others, like me, need a LOT of liquid. I sweat a lot and have to drink a lot. On summer rides here in St. Louis, where it's very hot and humid, if I go longer than about 25 miles (40 km) I add electrolytes.

Personally, I'd drink when thirsty up to the point where you feel like no matter how much you drink, youre still thirsty. That's when you definitely need electrolytes.