Once the chain stretches to a point (there are chain stretch gauges) it needs to be replaced. If you don't, then the stretched chain starts wearing into the gears, especially the cassettes. You'll know when the chain and cassettes are worn _ the chain jumps over the cassette teeth when power is applied. The trouble is, you may replace a chain and find that it now jumps the cassette teeth. Then the cassette needs replacing too. It gets a bit expensive if one does not check it regularly.Well, that's a pretty cold thought on my marriage! LOL! She may soon get into riding... I'm working on it. But what she doesn't understand is bike parts aren't car parts. Tires are only good for maybe 2500 to 3000 miles. Chains, cassettes and brake pads about the same. Cables a bit longer. My derailers and chain ring are all about shot, but they've had a good run. I'd have to check my maitenence records, but they all have never been replaced and it's been at least 6 years (which is at least 30,000 miles).
Personally, I clean my bike after most rides. I clean the chain and gears with a rag (de-greaser is the worst thing to use on chain and gears), re-lube the chain and wipe off lube with a rag (stops grit sticking to chain). Then I check tires for glass etc with a dental probe. Pump tires up, and recharge lights. If my ride is less than 40 kms, I usually do the routine after my next ride. Most guys don't do what I do; and it is these guys who get most of the flat tires (which I end up repairing, because they are so slow), and most of the visits to bike shops.
I feel much more confident starting a ride knowing that the bike is in tiptop condition, ready to push it to its limits.