Of course, you know I'm going to say, "That depends." (lol) It depends on the size of your reservoir, whether it's covered or not, how close it is to heat and/or light, and what kinds of plants you're growing, as to how often to replace the water. When it gets low, you'll need to refill it, but not necessarily replace it.
Take note of how much water your reservoir holds. When you add water, measure it and keep a log. As soon as you've added 1/2 of the tank size, then it's time to replace the water. In order to avoid shocking your plants, you'll only replace 1/2 of it with fresh, clean water. (Don't replace the whole thing.) Add nutrients.
If you need to clean the tank, take your plants out and pour out all the water, but keep 1/2 of it in a container. Now use a non-soap sponge to clean the inside of the reservoir. If it's really dirty, you can use baking soda to scrub with. Rinse it out, then pour back the water you kept, and top it off with fresh water until the reservoir is full. Add nutrients, then the plants.