I wanted to publish it in a poll form, but don't have the fiddling patiance to figure out how.
Here are my answers on our Mocha Co-Pilot, ridden mostly on pavement and some dirt road trips. This is our second tandem, first was sold with 3000 km, so not long enough to know the lifetime of most parts.
Drive chain: 3000 km
Drive chain wheels: not yet
Cassette: replaced but not on account of wear
Synchronization chain: not yet, looks like it might go about 5000 km
Synchronization chain wheels: not yet
Eccentric / captain bottom bracket: not yet
Stoker bottom bracket: not yet
Head set: not yet
Brake pads - rim brakes 3000 km
Brake pads - disk brakes: n/a
Brake shoes - drum brake - Not yet
Rear tire: Shwalbe Marathon XR: 5000 km?