A. DEF consumption will be approximately 2% of the diesel fuel consumed. Another way to consider it is that DEF will be consumed on a 50 to 1 ratio with diesel. (For every 50 gallons of diesel fuel burned, you will use 1 gallon of DEF).
If you know the average fuel consumption of a vehicle, you can easily calculate the amount of DEF that will be used.
Annual miles for average truck = 50,000 miles
MPG for average truck = 8 mpg
50,000 miles / 8 mpg = 6,250 gallons diesel fuel per year
DEF usage @ 2% of fuel consumption = 125 gallons of DEF / year
125 gallons / 10 gallon tank (average size) = 13 DEF fill-ups / year