When I had Rabbit/Golf/Jetta's I used the following milage rules for timing
belts:

 8 valve engines: 60,000 miles
16 valve engines: 40,000 miles

Water pumps would normally last a bit more, maybe 75,000 miles but were
driven by a normal belt so were never replaced at timing belt time.

Now that I have a Eurovan I have had the belt and water pump replaced at
70,000 miles. It appears that the Eurovan engine is a So Called "Free
Wheeling" design that can survive a broke timing belt without valves and
pistons getting to know each other too intimately. But considering this AND
the difficulty of replacing the Eurovan's timing belt I am using the
following milage rules:

Eurovan 2.5L gas engine

    Timing belt: 50,000 miles
    Water pump:  50,000 miles
Surpentine belt: 50,000 miles

While any one of these parts do not cost much when done at the same time
thay will add up to over $100 together.

Just remember: None of these parts are easy to replace on the side of the
road, at night, with no flash light, in the rain, with 4 whining kids and a
wife who is not happy either. This has not happened to me (yet) but keep the
image in your head when replacing this stuff.