I can’t speak to the overall lifespan of the battery, but I’m US based, and my understanding is that the battery is covered under the general warranty of the aids, which is 3 years. My audiologist recommended that right before the warranty period is up, I can come in and get the batteries replaced regardless of the current state. So in theory, at the end of 3 years, I’ll have fresh rechargeable batteries in there. I don’t know what the cost of a subsequent replacement would be, but assuming they last another 2 years or so, I’d likely be looking to replace my aids at that point.
Unfortunately, for me, just over one year in, the batteries have been the least of my worries so far, as one aid is in for a second repair or replacement already, so I have no real gauge on the lifespan of the batteries yet.