Welcome to the world of used i3 ownership ... I'm new here myself! (June 8)
For the sake of comparison: The 2014 i3 BEV had an advertised range of 81 miles with 100% battery condition. I am quite certain that this was accomplished in "ideal condition" testing (flat road, moderate temperature, limited accessory usage, correct tire inflation, no head wind, blah, blah, blah.) That's reasonably good for the small battery pack, in my opinion--though I wish BMW had launched with the 94Ah pack instead! There's something about triple-digit range that just ticks a box in my head ...
Anywho, I was lucky to find a LOW mile, one-owner, trade-in car locally that had been extremely well cared for, charged properly, recalls performed timely, etc. That said, my daily commute is approximately 20 miles over quite hilly terrain at a maximum speed of 45mph. In Eco+ mode, I am able to get just over 70 miles per charge ... though I am assuming the final dozen or so miles are legit as I've kept the charging above 10%.
70ish from 80ish is approximately 12% battery degradation over a 10-year time span. If that trend holds out for the next 10 years that I own it, it should still be capable of over 60 miles on a charge, which will still be viable for my use around the city where I live.
I will close by adding that how one drives has a HUGE impact on range, so getting as much regen as possible through one-foot driving is the way.
Prost!