3-5 years on average. I've seen a few go a little more and I've seen some die within the first year or two. Expectations should be, at best, about five years though in most cases. At that point there is likely a loss of coolant inside due to permeation, as well as a reduction in cooling performance resulting from reduced passage sizes internally from growth and gunk formation. The chemicals used to retard biological growth in a closed loop simply don't retain their potency forever.
At five years, if it's being used with hardware I care about, I'd say replacement is well warranted especially if you've noticed ANY reduction in cooling performance or even the slightest signs of leakage. And honestly, even if you don't see any of that, it's preemptively a good idea as well.