A fuge is a means of cleansing the bad stuff in the water and adding good stuff to the water. The macro-algea and live rock/sand help to filter the water. I currently have the CPR 12" hang on fuge on my 26g. I only use this and water changes to filter the water (no skimmer). I know of a lot of people who only run a fuge with no skimmer.
Now, here's the kicker........it's different for every tank. You have to take into account the bioload that is in your tank, size of the tank, etc. The best bet is to try it and see how it goes. Just keep testing your water to make sure the parameters stay in the good ranges. You might want to have a hang on skimmer handy just in case you need it.
Let us know how it goes.