Head gaskets can last a very, very long time, especially if the engine is taken care of. 616's are pretty low stress all things considered and the short length of the block and similar metals of both head and block are good things for longevity of the head gasket in general.

Barring a catastrophic overheat or something bad, they rarely just "let go" they give warning and plenty of time to plan for the job. Your most common tattletale is residual pressure in the cooling system or a mystery coolant consumption that can't be traced and all external sources are ruled out. I wouldn't lose sleep over it.

If the head comes off, it's a good idea to do valve guides and get a complete valve job done along with a skim to give a good flat surface. Again, not something worth doing until it's needed.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

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1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]