Just to be clear I am going to explain how long a smart watch battery will last per charge NOT how long a battery will last until it needs to be replaced.
Most smart watches are made with pretty good quality controls. Like any product, the manufacturer has to be competitive, so they must provide the best QA (Quality Assurance or Quality Control) they can for the investment they make. The same is for the battery, decent battery decent watch. Smart Watch battery life is actually pertaining to usage life per charge of the battery. Most watches and batteries are of good quality, however, the key factor here is the amount of mAh, which stand for Milliamp hours. When researching, smart watches prior to your purchase you need to look at the technical specs where is should show the mAh. A good rating of say around 230 mAh will provide about 15 days standby time and 5-7 days usage of various features. Just like a cell/smart phone, a battery charge will always be reduced the more you use its features. So, a good rule of thumb is to look at your smart watch just like you would look at your phone. They are very similar is operation.
When trying to decide on what watch to buy is like trying to determine what mattress to buy. There are so many name brands and just as many unknown brands but any mattress you buy regardless of the brand will probably be okay. If you compare the Samsung watch battery life and the Apple watch battery life to one of the watch names that I have on my website you have to consider not only the brand name but how much is the cost of each. For Samsung, Apple and many others you pay a premium price as compared to a no name brand but some time the no name brand outlives the name brand product. So, just keep in mind the amount you pay for what you get. After all, when Mr. Henry Ford started manufacturing cars if became a brand name. Then along came Studebaker Chevrolet and so on, but at first no one knew anything about Studebaker or Chevrolet. Just be diligent when researching smart watches. You may pay $400 plus for a series 5 Apple watch or buy a no name brand for around $100 with the same or more features than the Apple watch. If you damaged your Apple watch beyond repair then you would be out what you paid, say $500-600, or if you bought a no name brand for say $50-125 and it was also damaged beyond repair, you could afford to replace if easier than the Apple watch. Just like buying a phone, be diligent about your watch purchase.