Replace when worn. The springs should be good almost forever. However over time the front ends tend to losen up a bit. It really depends on how picky you are and how much you want to spend. What some people consider normal wear and live with it, drives others nuts.

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2006 CL500
2009 C300 4matic
1969 280SE
2023 Ram 1500
2007 Tiara 3200