I’ve only used the upper end of the forerunner line. But getting 18+ days from the fenix with regular sports use seems well above spec, they spec it with up to 57h in GPS mode and 18d in regular smart watch mode without sports activities.

I’ve never gotten more than a week tops from my Garmin watches, a handful of runs and it needs a recharge. And I need to fully charge it if it’s a long run or risk it dying on me while I’m running.