Like the others said, I would just check them at every spark plug change, and possibly clean them up. I'd expect them to last at least 100k, although you may find performance related issues as they wear.

For data points, the original distributor cap and rotor in my 1987 300E lasted 185k miles. I replaced it again at 260k miles, but only because I had a spare on hand and had the whole thing off the car to reseal the front timing cover. There was some wear, but no significant driveability problems.

The original on my parents' 1991 300E went 170k miles before it was replaced. That was a preventative replacement, and not the result of a part failure.