I just replaced an iPhone X last year with a 15 that was still on the original battery. From my records, looks like I got it in early 2018, so pretty much 5 years. Admittedly for about the last year, that usable battery was largely completely depleted by the end of the day and it couldn't last through a heavy usage day without a top up. Realistically, replacing at year 4 probably would have been the smart move, I was mostly waiting to see if the 15 was going to be USB C before deciding.