I have been using my Gigabyte water cooling rig for 1 year now. And before tha I had an aquarius II from thermal take that lasted me 2 years before I upgraded for better diamter tubing. And as far as I know it is still running. As long as you take good gare of you equipment I can say for certain you should get at least 2-3 years from a good water cooling rig.
"taken care of" just means keeping an eye on it. You will need to make sure that your coolant does not become cloudy, or algae begin to grown in it, and make sure there is no corrosion (water-wetter works to prevent algae and corrosion BTW). If you properly leak test the setup before hand you should not have any leak problems. But you do need to keep an eye on the coolant level. But some of the kits include alarms to let you know if the coolant level is low.
Just make sure you use a good coolant (ie. destilled water mix, Fluid XP, or the manufacturers coolant mix), and you should not have to clean the tubing out. If you get algae growth or if the coolant becomes cloudy you will need to flush out the old coolant and clean the entire system. But good quality coolant usually takes care of that before it can happen. Plus some manufacturer's coolants are designed to be non conductive. So if you get a leak you might not have any damage to your components.
Another thing to do is every 6-9 months it can be a good idea to drain the system and replace the coolant. I have done this many times and it is pretty easy. The only thing to worry about is you need a container to hold the old coolant and a way to dispose of it.
But the advantages to water cooling out-weigh the risks IMHO.