From my experience, maximum chain life is about 3000-4000 miles. Less so If you weigh over 185ish lbs.or so and/or MUCH less if you ride the chain dry. I realize a lot of folks will say they've gotten more mileage wise, but this is a pretty good average best case. After the chain wears, it wears out the cassette/freewheel to match the chain profile, and a new chain will only skip over this profile. Chainrings wear a little slower than cassettes, unless you just keep in in one ring most of the time. Sight unseen, I'd say you need a chain and cassette minimum. A good bike shop can/will measure your chain for free, but if you don't have one nearby, measure the chain with a ruler starting at one of the pins. Each pin should be exactly 1/2" apart. Over the course of 12", if the pin is more than 1/8", the chain should be replaced.