I will use it until it stops being viable. This used to mean “until the battery dies”, but I’ve also swapped batteries in phones not designed for it, and still had to get a new one as the batteries available weren’t OEM and so bad they basically didn’t work despite being new. Like phone always just turns off at like 40% charge. That isn’t “viable” in my eyes.

Now I’ve got a Fairphone 5. It’s only been a few years, but I’m planning on 10+ years if I can make it happen. In theory some new feature could get me to buy one sooner, but I highly doubt it. Hasn’t happened in the past either.