How long the Apple Watch battery lasts?

Depending on what you mean by how long will it last, my original Apple Watch (I got it the day it was released to the public without Pre-order) is still working just fine with the battery. My battery will probably last between 12-24 hours on a watch that age depending on how frequently you get notifications, what apps you are using on it, how often you have the screen turned on, and various other usage.

Here is a link for the Battery Service Pricing:

Apple Watch Service Pricing - Apple Support

As a general concept rechargeable batteries have a predicted service life estimated in the number of full charges the battery has received - so if you charge once from 50% that is counted as half a charge cycle. As the battery gets older (increasing charge count) its maximum storage capacity slowly reduces compared with original capacity when manufactured. From memory Apple considers a battery as needing replacement when its maximum capacity drops below 80% of new, and equally from memory the expected life for the AW battery is about 500 charge cycles. Considering my AW3/LTE has about 70% remaining after 24 hours (30% used) that would suggest the battery is likely to last about 1500 days. In round figures that's about 4-5 years, by which time the watch will be Old Technology.