How often you have to refill depends on the humidity in your area / room where they are being incubated... if the humidity is high you may not have to refill very often or even run it dry (in the spring when we get a lot of rain often the incubators will run at 50%+ with no added water)... in some cases if the eggs aren't losing enough weight/water you may even want to run a dehumidifier in the room.
After day 18, like you said, take out the turners, lay down the eggs, fill the channels (if the humidity has been low, you may want to add a sponge in a glass or something to help up it, also it is easy to add water to sponge) to get humidity up to 65% leave everything alone and hopefully just watch the eggs hatch!