You can charge Asus notebook until it shows you that the battery is 100% full,
do not worry,
there is no problem if you left it charging for a long time,
as the battery can not go overcharged.
In case the alert that appears when the battery is low is annoying you,
you can change the alert settings,
so that it gives you the alert whenever you want it to be,
like under an hour or before 10 minutes.
There is also a battery light in notebook,
that indicates when it`s currently charging and it goes off when the battery is fully charged.