A combination of manufacturer’s specs and experience. We re-charge all the regular used rechargeables once a week on Mondays, The ones with an expected battery life of less than a month. Then there’s another group that gets recharged on the first Monday of every month.
For the ones with replaceable batteries, we set the first schedule based on manufacturer specs for a replacement at 25% of battery life and then test when we get to that point to see if things are going as expected. The ones with replaceable batteries all get replaced on the first Monday of the month in which they’re due, but some are due every six months, some every twelve months, etc. (And, yes, there’s a spreadsheet and calendar notifications to say which devices are due for replacement that month)
It works pretty well for us. Other people might prefer to get battery notifications from the device itself and review everything once a week, but one of the advantages of using the schedule is that I can order the batteries in on and “as needed” basis based on the next week’s schedule. ( and, yes, calendar notifications for that too. )
So on Monday, my helper recharges the rechargeables. On the first Monday of the month, he also changes the batteries on the devices that are due to be changed that month. And I add replacement batteries to the shopping list for the ones coming up for replacement the next month if we’re low on anything. Monday is our “household maintenance” day. We do a lot of little stuff then.