how many years should an apple watch battery last?

I have a 3+ year old Apple Watch Series 4, and today, after 12+ hours, it has 77% charge starting from 100% charged this morning. I'll most likely still be over 50% by the time I head to bed this evening, when I'll put it back on the charger.

If you do not constantly run it into the Red (Li-Ion does not like being fully drained), and you do not have power hungry apps running all the time, it should last longer than my 3+ years.

But if you are on the cell phone all the time, or you spend time in an area with very weak cellular coverage so that the Apple Watch has to up its cellular radio power to stay connected (this is not an issue if your iPhone is nearby, as the Apple Watch will just piggy back off of your iPhone).

Or if you are into marathon or ultra-marathon races where the Apple Watch is running the GPS and heart rate sampling, and exercise tracking for hours on end.

Or you stream music constantly on your Apple Watch, away from your iPhone.

Or you have a 3rd party app that loves to consume battery.

Then your battery will deplete faster, and more often end up draining excessively.