My Apple Watch is staying charged for 8 hours

The Series 5 watch was discontinued in September 2020, so perhaps you bought one which sat on a shelf?

Either way, the battery should still perform better than 8 hours. Is your watch updated to the current WatchOS 9.4. Do you ever restart the watch? How do you use your watch, such as do you tend to stream music for extended periods of time. How one uses their watch greatly affects how long the battery will last.

And finally, what is the watch Battery Health? On your watch, open the Settings App. Scan up till you see Battery and check its Battery Health there. If you are near or below 80%, the battery is at its end of useful life.

What model watch and how long have you owned it?