The recommended level of charge for storage is 40%. You can check stored batteries every few months and charge them to 40% if needed.

If a Li-ion battery is stored without charge for a long time, the battery cells will lose their capacity irreversibly.

Globalmediapro batteries long-term storage specifications
Storage temperature : -20 to +25 oC (-4 to +77 oF)
Storage barometric pressure : 86 to 106 kPa (645 to 795 mmhg)
Storage humidity : 45% to 75%