According to a various reports the Battery should last around two days, though will likely need to practice some optimisation techniques. https://www.gsmarena.com/samsung_galaxy_watch3-review-2150p3.php https://www.guidingtech.com/save-battery-samsung-galaxy-watch3/ However I have not been enticed by smart watch myself thus far (just bought a new device anyway), if I was not lasting that long perhaps would question the effectiveness.
I do not work for Samsung or make Samsung Products but provide independent advice and valuable contributions.
My Device- S21 Ultra 5G ( SM-998B/DS ) CSC= BTU , 12GB/256GB
One Ui 5.0,Android 13 .