Thanks for all the replies, it's amazing how much interest timing chain threads generate. The responses were much as I expected.

From this I would conclude that for my family's current "fleet" the following applies:

My '73 350SLC W107 (3.5 M116, double row chain) could probably use a new chain despite the current one only being there for the last 120,000km and with 6 monthly (2,500 to 5,000km oil/filter changes).

My '90 190E-2.3 W201 (8 valve M102, double row chain) at 219,000km I probably need not worry about for some time yet.

Our '90 300TE W124 (M103) at 179,000km I need never worry about.

My parent's '93 E320 W124 coupe (M104) at 130,000km I need never worry about.

My parent's '96 E230 W210 (M111) at 110,000km I need not worry about for a long time yet.

My wife's parent's '99 C200 W202 (M111) at 60,000km no worries for a long time yet.

How about other related components? Does everyone replace just the chain or do they replace the tensioner and various rails and guides as well? In some situations I believe the guides and rails can be the actual cause of chain failure rather than the chain itself.

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107.023: 350SLC, 3-speed auto, icon gold, parchment MBtex (sold 2012 after 29 years ownership).
107.026: 500SLC, 4-speed auto, thistle green, green velour.
124.090: 300TE, 4-speed auto, arctic white, cream-beige MBtex.
201.028: 190E 2.3 Sportline, 5-speed manual, arctic white, blue leather.
201.028: 190E 2.3, 4-speed auto, blue-black, grey MBtex.
201.034: 190E 2.3-16, 5-speed manual, blue-black, black leather.