I usually charge it when I get back home. A few hours later, I put it back on to use as a sleep tracker. I then will wear it all day. So figure about 20-22 hours of use. With sleep tracking, I have the app running but turn the watch to theater mode. Otherwise I have Raise to Wake on and Hey Siri on. After all that, it's at about 80 percent.
It's the GPS-only model, so if someone has cellular on it won't last as long. But this is still insane. I don't know why Apple only advertises 18 hours of battery life.