I bought my X5 from a dealer in Maryland with a little over 71k miles on it. I was told that the DEF tanks were filled. A little over 3k miles later I got the warning:
Odometer at Low DEF warning: 74124
Date of Low DEF warning: 12/17/2014
Volume of DEF added: 1 gallon Active/3.5-gallon Passive

Winter 2014/2015 was exceptionally cold and snow filled winter.
Miles since last Low DEF warning: 11155
Days since last Low DEF warning: 300
Odometer at Low DEF warning: 85279 miles
Date of Low DEF warning: 10/13/2015
Volume of DEF added: 1 gallon Active/4-gallon Passive

I made a few trips down to and back from Virginia the last half of 2015 and into 2016 along with a few trips to and from Connecticut. I did not operate the X5 from February to May 2016. In July 2016 I took the X5 cross-country and operated for about a month at higher elevations and high-desert temperatures.
Miles since last Low DEF warning: 13062
Days since last Low DEF warning: 300
Odometer at Low DEF warning: 98341 miles
Date of Low DEF warning: 8/8/2016
Volume of DEF added: 0.5 gallon Active/5.5-gallon Passive

From the end of 2016 through most of 2017, I drove up and down the East Coast about eight times and also had about a 20 mile commute every morning for most of those months.
Miles since last Low DEF warning: 12984
Days since last Low DEF warning: 290
Odometer at Low DEF warning: 111325
Date of Low DEF warning: 5/25/2017
Volume of DEF added: 1 gallon Active/5-gallon Passive

In June 2017, the Active tank and SCR supply/dosing module were replaced. October through December of 2017 I did not use the X5 at all. My thermostat began to very. very slowly shows signs of failure starting early 2018. I moved in April 2018 and now have an 80-mile round trip to work every morning.
Miles since last Low DEF warning: 15918
Days since last Low DEF warning: 442
Odometer at Low DEF warning: 127243 miles
Date of Low DEF warning: 8/10/2018
Volume of DEF added: 2.5 gallon Active/3.5-gallon Passive

My thermostats were replaced October 2018.

I only use BMW DEF. I figure it will take five to six gallons at any one time to top them off. The DEF is used at all times during operation and not just during regeneration. The actual consumption rate will depend on air temperature, engine temperature, catalyst temperature, “throttle” position, engine load, exhaust temperature, etc., which all effect emissions and NOx output.

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8/2011 X5 xDrive35d Sport, Black Sapphire Metallic, Black Nevada Leather, Fine Burr Walnut Trim
2/2001 M5, Jet Black, Exclusive Complete Black Walk Nappa Point Heritage Leather, Black Cubic Trim