Wow look at you go!
If you aren't going to buy an auto top off, you will need a bigger return chamber or you will have to top it up everyday. The bigger the return chamber the longer it can go. If you get an auto top off (it's on of those things that once you do it, you never go back lol) then you can make the return chamber smaller. I use an auto top off and I like to incorporate a top off chamber into sump which holds the RO water for the auto top off to use.
I don't like to use a fuge, so I use that saved space for the above mentioned RO tank.