I leave my egos on the charger all night. 8+ hours every night. Haven't had any problems. I assume it shuts itself off when it's fully charged because during charging the charger is warm, and when the light is green it is cool to the touch.
I don't want to scare you, but you really should not do that. Most of the suppliers carry a Lipo-Safe Charging Bag. You can put the battery in there when it's charging if you aren't in the room. Best to not leave a charging battery unattended.
Although not super frequent, there have been reports of issues with ego type
batteries. The overcharge protection circuit fitted on eGo type
batteries can fail. Like
any battery.
Peace of mind, don't leave unattended. Or get the li po bag. Some people put their battery and charger in a metal baking pan. I think it was the year before last someone was charging their battery on a chair and the chair and her drapes caught on fire. Not trying to scare you and I'm not an alarmist type person. Better safe than sorry.