In general, good long-term lipo battery storage voltage is anywhere between 3.8v - 3.9v per cell for the batteries we use when flying FPV drones or wings.

You should avoid keeping your batteries charged and idle for too long. If you know you won’t be able to use them in the next day or two, it’s better to discharge them to storage voltage.

There are many chargers that support this function, for example I’m using this charger that can charge anything up to 6S and supports discharging and storage voltage.