Most water is lost
by gassing when the voltage gets above ~14.2V. Left on float below this voltage, almost no water should be lost
. The relationship between AHr consumed/replaced is directly proportional to the amount of time spent recharging above the gassing voltage.
For us full time cruising and daily recharging above gassing voltage, the batteries need water every 2-3 months. I check them every 3-4 weeks. Our bank is 675Ahr, from which we use 100-150Ahr/day. A bank that is discharged more each day will spend more time above gassing voltage and will use more water.