What's your kettle look like? Taller, narrower kettles, like a converted keg for example it's probably about 1 gallon per hour. Wide kettles it'll be more, say up to 2 gallons per hour.
If you want to dial it in for your system before brew day throw a few gallons into the kettle, measure at the beginning, and boil it for half a hour, and measure it at the end. That'll let you estimate how much you'll boil off on brew day.