what's the battery life of series 8 watch

If you mean - how log will your watch run before you need to charge it - there is no easy answer. It depends how you use the watch, if you are using the cellular function, and strength of signal from the cellular network, how much the display is used, even things like ambient temperature. As a general guide, Apple rates at least 18 hours under standard test conditions and most users seem to expect at least 24 hours usage (in other words, they charge once a day)

If you mean how long will the battery last before it needs to be replaced, Apple's target is that. the battery should still have >80% of original manufactured capacity after 500 full charge/discharge cycles.

Scroll to the second half of this article where it talks about the Apple Watch Series 8

Apple Watch - Battery - Apple

You should charge you Apple Watch every day (easiest is putting it on the charger overnight). If you do sleep tracking, the charge while getting ready for bed, and charge again while getting ready for the day.

You should never allow the Apple Watch to drain to the point when it is in the Red zone (few percent remaining). As that is the fastest way to destroy a Li-Ion battery.

My Apple Watch Series 4 is 4.5 years old, and now near the end of the day, I have 39% charge remaining, because I always charged it every night, and put the Apple Watch into Low Power Mode if I was ever in a situation where I could not get back to the charger for its nightly charge.