From my admittedly shallow knowledge of historical liquid cooling systems, i would think 20-30 years... I don't think the pump will wear out, since it's not a mechanical pump, and that's often what kills cooling systems. But think of it this way: the liquid cooling system in a window air-conditioner is much more complex, runs more liquid, is two stage, and has more mechanical parts; yet it's not at all uncommon for them to last 15-20+ years.
bottom line: something else will die on the G5s before the cooling system breaks down.
paul