My 2014 i3/REX gives 50-60 miles when the temps are in the 20's and 30's (with battery pre-conditioning), and 70+ miles in warm weather. In the winter I charge every night. Sometimes even during the middle of the day if I have been running around a lot and know that I will need to run around more. In the warmer weather I may charge at night and may not, depending upon my driving for the day. Overall, with about 8 months experience with the i3, my motto has become "if in doubt, charge". But since I have the REX as a backup, I am sometimes a bit cavalier about that.
2014 BMW i3 with Range Extender Engine, Capparis White/Frozen Black, 20" wheels,, Giga World, Tech + Driving Assist, Parking Assist, DC Fast Charging, TurboCord EVSE