Most rigs I've driven were 12-15 gallons capacity and 15-18,000 mile/changeover; Detroit, Cat and Cummins.
Most were also past 600,000 having each had over a half-dozen drivers, plenty of all-night idling (pretty much 6/nights week), no auto-shut-off or other gizmos besides fast idle ramp-up.
Consumption was generally 6-8000/mile per gallon topoff.
Clutches and transmissions were replaced before motors, most were auctioned off at 650-800,000 miles with plenty of miles left before an in-frame needed.
Smart companies and conscientious drivers can take 'em a long ways, even with no shut-off for 6 days week except to fuel daily; though, for many reasons, this is not "ideal".