I have been tracking my DEF usage pretty regularly. When I drive around town, or pull my camper I use much more. When I cruise down the highway, it seems to do much better. Which should be pretty obvious. Our trucks are designed to run on 3% DEF.
Lets look at some real world numbers though.
At 12,739 miles I had to fill my DEF again. I put in almost 2 full jugs (5 gallons). I'll call it 5 gallons as it was close enough. That puts me as adding 12.5 gallons total since I bought the truck new. Or about 1019 mpg of DEF. We should be burning about 3% DEF and 97% Diesel. So doing the math on it I should have burned only 416.67 gallons of diesel which would yield ~30.5 mpg of diesel. I wish I was getting that.
According to Fuel Log Pro over the course of 12503 miles (didn't start tracking until after my first fill-up), I have averaged 16.7 mpg. Extrapolating that out for the full life of my truck I have used 762.81 gallons of fuel total. This means that my truck has only been burning about 1.64% DEF which is half of what it is designed to use.
So why so little? My guess is that 90% or more of my miles are spent on the highway with cruise control set. I make a conscious effort to try and get the best economy as possible (I've made 200 mile trips where the display read 29mpg).