Let's see if I got this right:
Annual miles between oil changes = 13,000 miles
MPG for my 335d = 37.5 mpg
13,000 miles / 37.5 mpg = 347 gallons diesel fuel per year
DEF usage @ 2% of fuel consumption = 6.9 gallons of DEF / year
6.9 gallons / 6.1 gallon tank (average size) = >1 DEF fill-ups / year
My first refills were early due to a repeated issue with the transfer pump, but I've made it to the past two 13k oil changes without any DEF adds. I guess BMW's add rate is less. than 2%.