I know Apple suggest replacing our batteries when its 80% health or lower, but in reality, in witch battery health % do you see enough loss of battery life that a battery replacement is considered? My 13 PM is at 93% battery health and its still have decent battery life, but I clearly see its not the same as when I got the phone brand new.
This depends on two factors, in my opinion, knowledge, and experience: the iOS version and your requirements and tolerance.
I am running a 6s on iOS 10 with 63% health. Battery life is almost like-new. There has been a very slight decrease, but it is inconsequential. Too minimal for it to matter. I have screenshots from iOS 9 with a new battery, and I lost very little runtime with 63% health. Like I said, too minimal.
Extremely degraded batteries like mine, according to reports, are unusable on iOS 15 (and way before iOS 15, too), with many people with original, heavily used batteries like mine reporting both slowness due to throttling and abhorrent battery life, with the phone dropping several percentage points per minute, scrambling to get little more than one hour of usage... and unexpected shutdowns.
People have reported a significant difference if the phone is updated even with something like 85% health.
Battery health impact is none when the device isn’t updated, and massive when the device is. People with batteries like mine have repeatedly called the 6s “unusable” on iOS 15. A battery replacement brought that battery life to 3-3.5 hours. While not amazing, it is on the realm of usable.
I am not sure about this and I have no proof, but I reckon according to what I’ve read, that if the device is updated, maybe 100-85% is where it retains top-notch battery life? At the low-end of its supposed “lifespan” it starts to degrade. I assume it is because iOS 9 and 10’s efficiency on the A9 processor can counteract low battery health and those versions don’t ask the processor for the performance peaks that obliterate battery life. iOS 15 requires that performance peak due to increased power requirements, and asking that degraded battery to put up with a far higher power surge than it was designed for seems too much.
So, as a direct answer to your question: if not updated, it doesn’t matter. If updated, 100 to maybe 85% is good, anything below that and battery life plummets.
Interestingly, iPads aren’t affected: their batteries are too large for them to suffer like this, and they can cope with the increased power requirements. Yes, to be clear, battery life on updated iPads is abhorrent as well, but they don’t have the performance issues iPhones have, and even if battery life is far worse than on original iOS versions (in terms of percentage it might be even worse than updated iPhones with new batteries), it is good enough for the iPad to be usable. Moderately heavy users will probably have to charge daily, with 1st-gen iPad Pro users reporting anywhere between 4 to 7 hours depending on usage. Original versions of iOS hover around the 12 to 14-hour range. iOS 12 hovers around the 9 to 11-hour mark. So... chopped by 60% in many cases... but usable. Unlike heavily degraded, updated iPhones.