According to this
http://www.globalsecurity.org/milita...102/Ch6.htm#s3
a trench 40233.6 Meters (25 miles) by 1.5 meters by 3.5 meters would take 2 M9 ACE 603.504 hours to complete.
Since the formulas are for anti tank ditches I would assume that number would go higher if you are on anything other than smooth ground.
Base fuel usage for the M9 (109 per period*), two of them working for 603.5 hours would be ~32891 liters of diesel.
* Source:
http://www.pmulcahy.com/tracked_engi...les/us_tev.htm