Check the water level in every cell of each battery. It should be about 1/4" above the plates.
If you did fully deplete the batteries, the onboard charger might not be able to start. You’ll have to get a manual / old school charger and hook up each battery individually for a few hours or days depending on the charger output.
Once they are charged that way, you can use the onboard charger again. Do you know which charger it is? Originally, e825’s came with either a Schott charger or a Zivan charger depending on the year. If it’s a Zivan, the error codes can be looked up here:
As for how often to charge them, the spedometer display will alternate between MPH and the charge level as you are driving. When the charge drops to a new level, it will display it then go back to MPH. IIRC, it also displays charge when you stop for a moment. Typically, I wait for it to drop to about 80% or less to charge, I try to avoid running below 40% as this can damage the lead batteries.
You can also go by mileage once you get a feel for your driving patterns.