How often do shocks need to be replaced.
 

Just need a quick answer. My 400e has 215,000 miles on them and the shocks have never been replaced. Ive been getting a horrible sqeaking sound and im pretty sure its the shock seeing as though i have lubricated every joint with wd40 at least 3 times.

well, my 300E's rear shocks were shot at 192k. The fronts were fine when I sold it at 204k but they may have been replaced by the p/o.

try bouncing the car/looking for fluid leaks. Should be pretty obvious if they're knackered.

Replacing shocks at 215k on a 400E would not be considered wasteful or extravagant. If they are not leaking fluid it may not yet be necessity - there are certainly cases of 300k life. But 215k is at a stage where you will probably see a noticeable improvement in ride - with the added bonus that it may stop the noise.

If by Inspecting the shocks, you see visual oil leak marks on them, then shock need to be replaced. But squiking noise could vert well be some thing else. I was having squicking /grunting sound, thought it was shocks. Changed all four of them, the sound was still there. Later found out it was guid rod mount. I am not sure what you mean by "lubricated every jont" since all the joints are sealed on these cars and do not have lubrication nipples.
By visually inspecting if the boot on the guid rod mount is torn, that is the culprit. Another item to check for is, Lower control arm bushing. Take your time and trouble shoot, do not through parts on the problem.
Hope it helps.

Mine were still ok at 241k but they were leaking and due. It might not be a bad idea to replace them the car might ride a lot better.