Replacing Vehicle Shocks & Struts

When is it time to replace shocks and struts?

Replacing shocks and struts on a vehicle is an very large expense. When do I need to replace them at all? Unlike tires, the vehicle still can pass state inspection with leaking shocks. There is no real driving force to make one to replace them, except one's personal preference.

Last time I had to replace shocks was because all four shocks were completely out of commission. The truck was bouncing around, hard to control./steer, and and tires were worn pre-maturely.

If I plan to keep the car for 100,000 miles, do I sell the car as is with the original shocks?

If I plan to keep the car for 130,000 miles, do I ride out the "bad time'?

If I plan to keep the car until it dies (say 200,000), do I wait until the bad shocks had done damages to other components of the vehicle?

If I plan to keep the car for say 200,000 miles, do I change them out somewhere in between, says, 120,000?

A set of shocks and struts runs about ($40x2 + $75 x 2) = $230, and the installation and alignment $300 - is it worth the extra expense?