First of all, GREAT looking vehicle!
As for the DEF, I'm reading you'll start to see a warning light on the dash when the level of DEF reaches 29% with 1000 miles of driving left on the DEF tank. The DEF tank holds 5.5 gallons and may not have been filled completely when new, just like the fuel tank. However, your dealer may have filled the DEF.
How many miles you can go on a tank of DEF depends on a lot of factors, but you should average around 6000 miles per tank of DEF.
Enjoy that new truck!
