How much water evaporates depends on the difference between your water temp and your room temp, the humidity levels in the air, the surface area of your tank, the amount of water movement/flow, whether your tank has a lid, whether you're using fans, and a host of other things.
For what it's worth, I have to top off at least 6 or 7 gallons per week (often more) in my 150 gallon tank, but that might be more or less than you depending on how we differ on all the variables noted above.