I also have not found any i3 Rex spcecific maintenance schedules other than the oil and filter change every 12 months. However, the Rex engine is a detuned BMW C650 scooter engine. The maintenance schedule for the C650 shows the spark plug change interval to be every 24k miles (40k km).
I calculated that running the Rex gets me around 34mpg. Our tank is 1.9USg (2.4USg if coded).
At 34mpg, you would use 706 USg to travel 24k miles.
So basically, to follow a distance based schedule, the spark plugs should be changed every 372 tank fillups (294 fillups if coded).
I have owned my 2015 i3 Rex for 4 months, filled it up once and still have 3/4 tank of gas. So based on this, I'll be due a spark plug change right around the turn of the Millennium
AFAIK, this engine uses a cam chain, not a belt. Given the expected duty cycle, this should last well beyond the serviceable life of the actual car.
Basically, I have decided that my maintenance will consist of oil every 12 months.