I have heard of someone listening to the advice of his mechanic about not needing to replace the water pump because... "a water pump either works or it doesn't" and having the timing belt done, and 15k miles later having to repeat the process because the water pump finally crapped the bed.
From advice giving to me, they should be changed every 90K miles. If you have it done, which you should, by 90K, then it will be wise to do it again by 180K, 270... This is provided you are driving the car in non-extreme conditions. Extreme conditions of course require shorter maintenance intervals.