You can charge your battery whenever you need to, no matter how full or empty the iphone extended battery is. Lithium batteries -- which the iPhone uses -- have none of the adverse memory effects of nickel-based rechargeable batteries.
It is OK to let your battery charge overnight, even if the battery will be finished charging halfway through the night; the battery does not overcharge.
Lithium batteries have a limited number of charge cycles; while your battery should last for at least 400 charge cycles with proper maintenance, you will eventually need to replace the battery in your iPhone, also such as JIAYU G3.
Remove your iPhone from its carrying case when charging; certain cases can cause the iPhone to overheat during the charging process.