Shock Rebound Rate
 

New M-B front shocks for 92 300E have a piston rod rebound rate (from full compression to full extension) of less than two seconds.

Is there maxiumum allowable time limit for rod rebound, beyond which a shock is considered to be worn and no longer usable?

from my experience with working on various cars - if the shocks rebound once and stop - they are fine. If they rebound 3 or more times they are really toast.