I'm not sure where you came up with that. That would mean I have to buy a new chain every 12-20 weeks. That ain't happening. My chains last many times longer than 4K miles. And before I buy a new chain I inspect the cassette with a magnifying glass. The teeth are almost never worn. A cassette will last me a couple of years, and I ride almost everyday in all weather. Chainrings aren't even on the radar. Just keep everything cleaned and lubed.

If a shop told the OP he needs a new chain and they didn't even inspect it or check the stretch then they're just trying to get his money.