I guesstimate my battery is about 85% now. But let's say 80% if you want to be super conservative.
It was an orginal battery that was eligible to free battery replacement (S/N checked out when Apple released the news), but I never availed myself of the battery replacement for fear of a poor/middling job of them putting it back together again. Plus, the screen protector I put on it when I first got it is perfect-as-can-be and I was loathe to give that up.
I always was sure to run the phone in low-power mode (the 6s was the first model to debut that feature, I believe) which may have prolonged its life.
As it stands, I'm fine staying with this phone for another 2+ years more if necessary. I may buy a new iphone in 2021 or 2022...a lot really depends on if the notch is still there or not.