There are alot of variables that seem to go into the usage. Weather/traffic/driving habits/idle time. Plus it's hard to say if the DEF tank was truly full when you take from the dealer, since there is only the "block meter reading"
I have 9300 mile's on mine. living in metro detroit, so it's remote start winter season. I put in 2.5gal myself around the 7500 mark, and the dealer refilled it during my oil change at the 4000 mark. Currently sitting one bar above half. 95% of my weekly mileage is city driving.