a very senior member here would say until it won't pass fuel anymore..
for me i changed a filter that appeared to be good in the end (the canister type) and have been running it's replacement for at least 4 years.. with no issues.
of course a w123 has the clear prefilter to be able to judge things fairly easily.

when in doubt change it as they are cheap insurance and keep a spare in the trunk!

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1984 300D Turbo - 231k....totalled 11/30/07 RIP