Let's say for the mid-line 105/Ultegra or Veloce/Centaur components, how long do each of these last? Give me 2 figures- the first, when you would suggest replacing them, & how long they can still be usable.
And static components like handlebars, stems, seatposts, saddles, etc.
NOT frames & wheels, I'm not going to open that box.
Kirk Frameworks JKS-C, Serotta Nova, Gazelle AB-Frame, Fuji Team Issue, Schwinn Crosscut, All-City Space Horse
I've most often heard that replacing the chain every 1500-2000mi will greatly increase the milage you'll get out of the chainrings and cassette. Otherwise, if you go 4000+ (ish depending on maintance) on the chain, the other stuff will likely need replacing as well. As with any mechanical system there are many variations that will cause high and low values for wear life.
aka: I've never had a bike long enough to need any of that stuff replaced yet that I actually bought the parts new...
Drivetrain lifespan depends on a lot of factors, including how clean and lubed you keep your chain, how much you ride in the rain, how hard you pedal, and how much you weigh. A chain might last 1000 miles under real harsh use; maybe 5 times that under nicer conditions.
Your gears will last a lot longer if you regularly check for chain wear and replace the chain before it gets significantly worn. Some people will need to replace their cassette every time they replace their chain. Others can go through 3 or 5 or 10 chains before they need to replace the cassette.
measure chain pin to pin over 12 links when its taut, if its more than 12 1/16" you went too far, less than 12 1/8" and you may get away with just chain replacement, this is more critical with cassettes and even more so if you tend to use ony a few gears which is many many people out there---a new chain will tend to skip if the previous one was left on too long allowing the teeth in the cassette cogs to get oblong patterns in them
so replacement mileage will be your average when you get to 12 1/16" for 12 links, singlespeed bikes can go much longer--often better with those just to let them wear together and replace all of it at once
BB's will vary, wait till you feel play in the bearings and replace it before it fails, to test for this grab a crank arm and try to wiggle it side to side on the bike around various points of its rotation, it will get loose around 12 and 6 o clock first usually, ive been seeing around 7-7500 miles or so on mine or about 6 months, wintertime seems to shave a bit off their life but not much considering all the salt and slush and water I ride thru. They sometimes will go quite awhile with em a bit loose before they die, but once they start creaking beware, they will die shortly after that.
Chainrings are good for quite awhile, as long as the chain will mesh enough not to skip or wear the chain badly your ok, look for abnormal signs of daylight thru the chain/chainring and/or big oblong wear patterns in the chainring teeth. Chainring on my work bike has like 24,000 miles on it and its fine. Steel wears longer than aluminum too.
Static items are usually good for most of the bike's life. My workibike has its original stem, seatpost, handlebars, headset, and single brake, all of which are about 25 years old and have seen more than 60k miles.
Saddles wear out, weather and use does them in sooner or later, and mine always seem to be like sneakers, they are at their most comfortable about 2 seconds before they disintegrate.
Cassette hubs, need cleaned and lubed when cassette is replaced, thats easy preventitive medicine, they skip when they go bad and/or the bearings will start making noise and feel rough.
But seriously, look at the classic and vintage forum and see how long this stuff can last. Far longer than most of us want to keep riding it I look at the guy a couple doors down from me that's riding some 40+ year old bike, with original components, I think he said he's on his third chain, and he rides the darn thing everyday! Take care of it and it will last just about forever.