When should the suspension be replaced?

The suspension is a system made up of dozens of parts. You don't replace the entire system; you replace worn or broken parts. Ford has always had much more trouble with parts breaking leading to loss of control and crashes than other manufacturers so an annual inspection at a tire and alignment shop is a good idea. Also, never ignore a squeal or clunk. Those are signs of a worn part that may be ready to come apart. The system should be inspected right away when you hear noises.