You know deep down that there's nothing you can do to stop these batteries being fairly useless three years down the line...
I would expect longer that, although it’ll obviously vary by usage. They won’t be comparable with the very short lifespan of in-ear Airpods in this regard. They’re tiny batteries in the first place, any reduction in health has a very significant hit on battery life and their ability to hold a charge.
At a guess, I’ve used mine cumulatively about 3 hours a day since Tuesday. I’ve got 42% charge left. At this rate, I expect I’ll be charging them once every 5 days or so. Inexact, but lets say 75 charges a year. I’d expect the battery to only start degrading from use after about 200-250 cycles, and even then I’d expect 15 hours plus.
Given any Li-Ion battery ages, used or not, the useful life of them will probably reach 5 years in my case. I’d expect them to be at least 75% health then, 15 hours or so. £75 for a battery replacement, I can live with that. (UK battery replacement priced at £309 was thankfully an error, Apple have confirmed it’s £75).
My old Bose QCII’s still have about 15 hours life after years of use. Even with no off switch, I expect the APM to be comparable.
It’s ingrained in me to not like the absence of a power switch, but so long as the battery have a useful life of 4-5 years, the upside is worth it. I love just putting them on and they’re ready. I only put them in the case when I go out or go to bed.
They’re ill suited to rare/occasional headphone users though I think. It would annoy the hell out of me if I picked them up and they always had low battery. But then you have to wonder why an occasional user would spend £550 on headphones, heh.