recommended maintenance is a teardown every 6 months (on distilled water + PTNuke). when you fill the loop, you may have to top off the res every now and then for a good week or so, but after that it's pretty much set-it and forget-it. The thing about watercooling that you should keep in mind is that it's a hobby, if you're getting into it for just a next step from air, you very well may end up miserable...do it because you want to, not because you feel it's necessary. That said, it's a hobby! Have some fun with it if you do get into it, fawn over your loop, hang out with the guys on this board, talk about stuff that most people don't care about, get into heated arguments and flame wars over distilled vs fluids...just generally enjoy yourself.
EDIT: Just realized, i didn't see closed loop...there's no closed loop 120.3's, coolit has a 120.2, but if you want a 120.3 rad you'll have to mod, then it's not closed loop, then there's no reason for you to have bought an inferior product.