Agreed with Jokinn, if an exchange is available as an option, that may be the safest way to go. As for when batteries fail, in short, anytime. It may be DOA, a week, 6 months, 5 years. I think that's a crap-shoot at best.
Just like a cell phone or car battery, sometimes they fail prematurely. The battery on one of my Blackvue cameras was 4 years old when I upgraded the camera. As far as I know, it's still going strong with my brother-in-law (to whom I gave the camera).
By contrast, my 6-month old 2015 Toyota Highlander was just in for routine maintenance, and they discovered that the battery had a failed cell. They replaced it (warranty, of course).
As I've mentioned in other posts, I have a number of the 0806's (9 of them) - they're all about 4 weeks old at this point. I've had an issue with the GPS mount on one, and a battery that has failed on another. With those two exceptions, everything else about these cameras has been spectacular. Granted, I've only had them for a month. Come back in a year, and you may find me cursing the brand. Who knows.
To send a camera back to China for me, costs about USD20.00. Since I bought mine for cheap from a no-name seller on AliExpress, I didn't expect to have much service. However, when my mount went bad, I contacted the seller and they immediately sent me a new one. They didn't ask for the original one back. When the charge on one camera would last but for 3 or 4 seconds after being unplugged, they sent me a battery, no questions. They did offer that I return it for a new camera, but I don't mind the technical challenge, and I didn't want to wait another couple of weeks to get it working.
I actually ordered a replacement battery from eBay to get it quicker. It was US8. When the other one arrives from China, I'll store it as a spare for when (not if) one of the other batteries go. Chances are, by then, I'll be ready to dump these in favor of the newest 4k (or 8k) cameras π