Most chargers stop charging when the light turns green, so it doesn't matter how long you leave it on the charger after that, it will not continue to charge. That applies to lithium battery chargers. NiMh and NiCad chargers may continue to charge as long as they are connected. The NiMh chargers I have, switch to a very low trickle charge after the green light comes on.
You really don't want to over-charge lithium batteries, even if you could. Thats when things start getting a little flaky.
So, I say, charge 'em til the light turns green and vape away.