There are different schools of thought on replacing components, so you have to figure out what camp you belong to. Some people want to get every last inch out of their components and are perfectly willing to keep them going even if the performance drops off a bit. Then, the other camp wants things to operate perfectly and they replace components before their performance goes downhill one bit.
In my life in cycling, I believe there are a lot more people who try to keep components going than there are those who replace them before they’re truly worn out. But, in that smaller camp are really serious riders who rely on perfect performance, like racers.
Lots of cyclists actually enjoy working on their bikes and solving mechanical problems on the road. And they’d rather ride their bikes than work on them as often as maybe they should. That’s the old, “don’t patch the roof if it’s not raining” theory.
So, I think it really comes down to your personality. The good thing is that even the cheapest bike parts last a long, long time, and even with pretty sloppy maintenance. And they also function pretty well even poorly maintained. So, the average rider often has no idea that his chain and cogs are completely worn out. He just keeps happily pedaling away, just like the newbie riding on a dry chain – squeak, squeak, squeak. They don’t even hear it. They only notice the difference when their bike is fixed.
Still, if you wanted to make a timeline of when parts should be replaced, you could do that, but to be accurate it would need to be based on your actual experiences with the parts factoring in the way you ride, the conditions, and where you ride, too. The “guidelines” and “rules” we mechanics give out are really just ballpark, not accurate for every rider. And bikes have improved a lot, so they require less maintenance than they used to, which is a great thing.