The Lithium Ion charge cycle does not allow for "overcharging" or extended charge times. The chargers do effectively shutoff, even though there is a minute amount of current still flowing.
When you start charging a battery, the charger starts in constant current mode. It will delivery a fixed amount of current until the battery voltage reaches the threshold at which the charger will switch modes. This threshold should ideally be 4.2v, but many chargers switch modes a little early. At that time the charger switches into constant voltage mode; only providing enough current to hold the battery at full voltage. This is also called the saturation charge. If you were to remove all current the instant that the battery reached 4.2v then after a short time the voltage will drop under full charge. Saturation is needed to ensure that the final charge voltage will not fall off. The charger will adjust the current in order to maintain ~4.2v. Eventually the current will be low enough that it is not worth continuing until "zero" current; diminishing returns. This is usually at around 10% of the initial constant current amount. This is the point at which the light turns green and the charge current stops. No additional time is needed, the saturation charge is complete. It is ready to vape.
All chargers that I have ever seen measured have a small amount of current that will flow after the light turns green. By small I mean usually less than 250uA (0.00025A). This current is irrelevant and would take months, or years, to have any effect on the charge, and that only if there was ZERO self-discharge.
At what point to charge the batteries?....I have a couple if lengthy posts on the subject that I think add a little logic to the subject vise just measuring life in terms if # of charge cycles. Search "partial discharge" to find them.