You will have to get a feel for it...like I check levels every PH adjustment and fill as needed. So every 3 days or so. When she gets bigger, she will get even more thirsty so it’s hard to put a date on it since it is plant size and reservoir size dependent.
Just make sure when you replace a gallon of water, you replace a gallons worth of nutrients too. Then change the water out completely every other week or so.