Don't buy Apple. Or Google.
My first smartwatch was the first smartwatch to look like today's, the LiveView put out by Sony in 2010 (battery life 4 days). It was appalling, and I soon gave up on it. Next was the Nike Fuelband in 2012 (battery life also 4 days), one of the first fitness trackers -- handed out by Autodesk marketing to us members of the press for free -- whose rubberized (and unswitchable) band irritated my skin.
Then came the winner, the Pebble Steel. Its claim to fame was to use digital paper for its display, so that the watch drew very little power while displaying watch faces; digital paper consumes power only when it changes the display. The watch lasted up to ten days between charges, and when I turned off Bluetooth, it eked out two weeks. But the display was coarse, and when FitBit bought Pebble it dropped support, and finally the display went bonkers, sadly.
Watches with operating systems from Google and Apple now dominate sales, unfortunately, proudly boasting that they last up to a day, if your day is 18, instead of the standard 24 hours.
The fundamental flaw with watches running WearOS and iOS is that they have big operating systems shoe-horned onto very small devices hosting tiny batteries. It's that "let's get a foothold into the market" thinking that has given us technological marvels that don't last much past bedtime.
But now some watch manufacturers are doing the hard work of writing new smartwatch operating systems from scratch. The new OSes minimize use of the CPU to prolong battery life to a week or longer. Mine, for instance, uses about 15% of the battery after 2.5 days.
I only really want a smartwatch to make it easier to change the time between time zones and light up in the dark -- for the convenience. I really don't care for all the other features. So the battery life in my watch is extended partly because I have the display on for just five seconds and turn off every function that I can -- sports, sleep, heart, stress, et al.
Here's the thing, tho: all the manufacturers writing efficient OSes for watches are in China: Motorola, Huawei, and a third one whose name I forget.
So as watches run out of new functions to add, the next battlefield will be longevity, and here China has the lead.
Update: Garmin also uses its own operating system in its watches to make them last a week or longer.