It takes about 2 hours to recharge a completely discharged battery.
You do not have to wait until the battery is fully discharged to recharge it. You can recharge the battery more frequently for a shorter charging time to ensure that the battery is never completely discharged.
*credit to Sylvain of iHealth Labs EU
https://helpcenter.ihealthlabs.eu/hc/en-gb/articles/205766712-iHealth-AM3-Time-to-charge-the-battery