Apple says that in general use the Apple Watch will last around 18 hours between charges. This figure was arrived at from tests in which an Apple Watch was paired with an iPhone and used for a variety of typical uses: 90 time checks, 90 notifications, 45 minutes of app use and a 30-minute workout with music playback from Apple Watch via Bluetooth.
Our experience suggests this might be about right for the original model, provided you don’t go overboard with app use and notifications, but (despite Apple continuing to use the 18-hours figure) the Series 2 and Series 3 models were capable of a lot more. In tests, the Series 2 managed 54 hours (including two nights, and fairly light typical use), while the Series 3 lasted for 39 hours (including one night). The Series 4 and 5 should last somewhere between 24 and 36 hours, although this will vary depending on your usage habits.
Whatever your typical usage, you can expect to have to charge your Apple Watch overnight at least every other night, and every night if you use a first-gen or Series 1 model, or if you use the cellular function a lot.
If your device is doing significantly worse than the above figures (and is new – battery performance will degrade as it ages), it’s worth looking into why, and trying to improve matters.