not really much at all
In a closed loop system i don't think you need that much mainaince. I'd turn the comp off if you're going away for a while, but other than maybe adding a little water every now and then, there shouldn't be much maintence. A critical temp shutdown can help to ease worries about pump / fan failure. If you're using a setup with an evaporative cooler then you're going to want to check on water levels daily, especially in warm weather with low humidity, but thats not really 'standard' water cooling either.
Almost forgot - The only other possible source for maintence is if you are mismatching metals in the system. For instance - an aluminum radiator and copper block. Ions will travel through the water and deposit on the block. You will need to clean out the block if this is the case. There are addatives such as water wetter and others (do a search for battery effect) that will reduce this chemical reaction, but the best way is to go all one metal (copper). I've never had a mismatched system, but my guess is that once a month or so you'd want to check the block out (more if u got a clear top) or when temps start going up and you can't explain it.