If a battery is filled full before charging, it very likely will expand and vent out the tubes. Never fill a battery completely full unless it is near full charge.
When a battery is low on fluid, it should first be filled to just above the plates, or to the low line, then charged to like 80-90%, then add a little fluid if needed, then fully charge.

If it is not a true maintainer float type charger, 13.2 volts max, don't leave it charge if its venting out the tubes, as it may be overcharging.

On edit, the 13.2 float voltage is a safe voltage in warm temperatures. In colder winter temps, a higher voltage (as high as 14v) can be used to maintain/float charge. If the voltage is in the proper range for the temperature, very little if any water usuage will take place,....check twice a year at most.

In a perfect world, we would all have a very expensive temperature compensating charger to maintain our batteries, which would vari the voltage as the temperature changes.