As long as you set it up correctly, they last just as long as a normal PC. You just have to be extremely careful when switching parts. You have to completely dry it off first if you ever plan on upgrading anything - which can be a pain in the ass since mineral oil is terribly difficult to get rid of.
You would need to make sure you get a pump strong enough to push the oil (since it is thicker than water) and a radiator large enough to dissipate the heat from the quantity of oil you're using. If you plan on doing this, just remember mineral oil expands when heated, so don't fill the tank all the way up.