We tapped into our pipes in the utility closet in the basement and routed a pipe for our rodi. I have a small one. only does 35gpd. I keep a 25 gallon rubbermaid tub full.
I was having to top of just 1/2-1 gallon /day, but we've just cut up some acrylic to form a cover in the sump, and now, I'm guessing I'll be doing top offs at 1-2 gallon/week. I guess my lack of evaporation is also since I don't have mh lights, I have my main tank covered 90% with glass, then the light just sitting on top. So less chance of evaporation
I time my rodi refill to coincide w/ water changes - fridays, I'll fill up the rubbermaid, by saturday, it'll be full, so I'll stick the heater and powerhead in it, then by sunday, the water is perfect temp and I can do water changes. When I'm done, I usually have enough left to do top offs. and emergency water changes for the week. Otherwise, if I run out, I'll have to wait a whole day before I have enough to do a massive change in emerbencies.