Yeah, how much of the surface area of the water is uncovered? That's the key factor. I have a tank with near 100% coverage and it loses almost 0 water. I could really only be measured in gallons per year loss... which might be 1-2 gallons of water evaporation a year if I had to guess. I never perform water changes on it. It's just stable.
My second tank is completely covered in the 75g display, but the sump is entirely uncovered, the sump being a 29 gallon. That seems to be losing maybe a liter a day. I may partially or completely cover the sump to minimize its water loss, but building a cover around the drain lines will take some time to figure out. I will probably need to find some kind of plastic sheets for that side and cut it to the exact shapes needed.
My house has AC running and a dehumidifier in the basement where the tanks are... My tanks run at 80 degrees.