This is a Wear OS question and there are a few apps that can mod the default setting. Watchmaker is another one that allows you to set how long the screen stays bright before it goes dim or off. Watchtiger is right, the longer the watch is in bright mode, the shorter your battery will last.
I found a face (and made some mods) that has what I want in dim mode so I rarely need to go bright unless it gets a notification or I access an app. I took my watch off the charger just before 7am and, at 4:45pm I am at 60%.