I have a '13 Dodge Dually 4x4 longbed with 2400 miles on it. My DEF gauge is still above 1/2 but I have not towed my camper yet or taken on any long trips so I have not put any extra hard driving on it.
I have heard DEF in the 2 1/2 gallon jugs have a long shelf life as I got a free one when I bought the truck 6 months ago. We are set to go on some long journeys camping so I will put that jug in the back of my truck just in case. If you run out, you go into limp mode at 5-10 mph.
IMO, I would take it back to your shop and use it on your trucks as you said you will not be driving your truck very often.