I would charge it once the light around the fire button begins to show red, indicating that the charge on the battery is getting low. While charging this light will be green and will stay "on", then will turn off when it is completely charged.
You can put the MVP on the charger at any point, you don't have to wait for it to discharge to a certain level. In fact, you can prolong the life of its battery if you charge it before it becomes low on a charge (called "topping it off").