Digging the hole took a full day with two tractors at time or 1 tractor and a man on the laser level. Day two was putting in the 6" of gravel and then laser leveling the pipe. Also on day two the pipe was covered with 2" of gravel with tractors riding on 2x12 lumber. Got it all the way to being ready for inspection on day two leaving just the header uncovered.
It took about another 6 hours with two tractors to carefully backfill using heaving 3/4 plywood with 2x12's on top of that to backfill. Had one tractor running on the lumber in the pit and another dumping dirt in from the side.
Not counted in all of this was the time to add a effluent filter at the tank outlet, fine grading with rakes, and dealing with buying all the pipe and doing engineering drawings of the layout.