You don't actually drain them to 0%. Once they drop below a certain voltage (forget what it is...2.8v?) they shut themselves off. This is to prevent overdraining them. However, their ability to product vapor falls off quite a bit before that, so when they start getting anemic, throw them on the charger and grab a fresh one.
Also note that LiIo batteries have no "memory" issue, so it doesn't hurt them to recharge if they aren't fully drained. HTH