I have a 2002 Dodge Caravan. When should I expect to replace the shocks & struts?
That depends entirely on your driving and the roads. You could drive on the interstate system for 500,000 miles on the same struts/shocks, or replace them at 40,000 miles operating in the High Chaperral back country.
On our Nissan with 96,000 miles they are still the original.
That’s very hard to say. Some cars 20,000 miles, some 100,000, some more. It depends on the road conditions, driving habits, and quality of shocks/struts. They can wear out gradually, so you might not be aware of it. Find a mechanic you trust and have him check them out.
When they wear out or start leaking you should replace them. Until then don’t worry about them.