My series 0 battery lasted less than 2 years. With AppleCare+, they replaced it for free. The replacement quickly degraded, similar to what those posting here described of their own watches.
My series 4 battery is at 93% after 2-1/2 years. I try to not charge it past 90%, and always turn it off at night. On my next watch (some day), I'll charge it twice a day, to not past 80%. That seems to be the point most experts agree is ideal. It doesn't matter how many times you charge it. What matters most is how many hours it's running, and how fully it's charged.
I've been reading a lot about Apple batteries during the past few months. My MacBook Pro started bulging, which got me to pay more attention to the subject. At first I chastised myself for cancelling AppleCare+ a few months after I bought it. (Apple closed all their local stores, so I saw no reason to keep it.) But then I realized AppleCare+ on my Mac would have expired before my battery went bad. Paying $200 for a replacement was less than what I saved by cancelling AppleCare+ on it, so I'm still ahead. I know $200 sounds like a lot, but there is much to consider. Apple also replaces the keyboard and trackpad when the battery is "serviced". More importantly, after-market batteries are substandard. Most go bad quickly. A warranty isn't of much help, since you'll get another substandard battery when the first one goes bad. Not to mention how dangerous these batteries can be. You don't want one of these in your house, let alone in your pocket or on your wrist. Manufacturers like Apple have reputations to uphold, so they try to get quality batteries. Even so, many MacBook batteries have gone bad like mine. Some even caught fire. Luckily for Apple, that happened at the same time Samsung was getting all the bad press for their phones catching fire. The lesson is clear: If you plan on keeping your device for a lot longer, you want a quality battery in it. Don't scrimp.
P.S. The reason I'm keeping my old MacBook is because it's my all-time favorite computer, and it's likely to last a whole lot longer. I just got through upgrading the storage when the original storage died. (Newer MacBooks have permanent storage which cannot be replaced–just throw your whole computer away when it dies.) I also took the opportunity to upgrade to 2TB when I did. Storage tends to last much longer if there's a lot in it not being used. That's likely one reason my original 256GB went bad so quickly. I kept it almost full.