All I can tell you is from my own personal experience. My very first smartwatch was the original Moto360. It was ... um ... not great with battery life. Maybe 10 hours on a good day and less as it ages. After only 18 months I was lucky to get 5 hours until the battery crapped out on me.
The Fossil OTOH is great. I put it on around 8 a.m. and by the time I take it off 12 or so hours later, it is still showing a 50% charge, and I do use custom watch faces (which can drain batteries faster ... sometimes), and check messages all day long (and that's probably in the neighborhood of 100-200/day).
Mind you this is a piece of technology and any watch is subject to bugs and glitches. I have had a few instances where a process gets "stuck" on and it drains the battery exceptionally fast and always before I realize it's happening. A reboot of the watch generally fixes that. Murphy's law of technology states that the more important the message, the more likely it is to happen.

But, in general it's been a very reliable device.