BMW indicates oil and filter change every 12 months on the Rex. If you are still under factory warranty, I'd stick with that, as you could have a warranty claim denied if you aren't following the proscribed maintenance. Out of warranty, judge it on use, but for me, I will be sticking to the one year schedule, as even with little use, the oil will absorb water as it sits, and start to deteriorate.
Seems expensive, but on my ICE car, I had to have the oil changed at least twice a year, sometimes three times a year, which is as much as or more than having the oil changed in the i3 once a year.
And you can always change the oil yourself:
2015 i3 Rex, Capparis White, Tera World, Technology & Driving Assistant, Parking Assistant, Harman Kardon Audio System, 20 inch wheels, EVoInnovate EVSE