I held on to my 8 Plus for little over two years and it remained excellent most of the time -- although I had to replace the battery before reselling it because it was at 83% capacity.

I also always charge my phone overnight even if it's at 70% (which it routinely is) because I've been bitten by going out and about with 60% charge and having to spend 12h outside and my phone died. So I just started conservatively charging it each night no matter what I expected for the next day.

But yes, for most people an iPhone can easily last two full days and still be at 15-20% when you finally plug it in on the second day's end.