Silverado DEF engine penalty progression?

[JFoy]

So, I just started working for a local Chevrolet dealership doing IT and support work. Having no background in the cars and trucks, I learned that the diesel trucks now have the DEF incorporated on them. Naturally, I asked what happens if you run out of fluid and was told the engine performance would begin to degrade in a series of steps until it simply shuts down. I think it is something like the top MPH the truck can go starts dropping over a couple hundred miles and progressively gets lower and lower.

Question is, does anyone know the progression steps, e.g. -10 MPH per 100 miles or something like that?

Thanks,
JFoy