Trying to dig a 12 to 30 foot long trench manually will be a nightmare. My recommendation is to bite the bullet and rent a trencher to do the work for you. Even at 12 inches, your work is cut out for you, but 36 inches is really deep.
After the two-month incubation period, the turtle hatchlings all dig out of their nest at the same time, a process that can take several days. As a group, they then leave their nest at night and head directly for the sea. This first trek imprints their home beach into the hatchlings.
Confidence votes 105. It takes about 21 days (3 weeks) for a chicken egg to hatch. 21 days exactly when they started incubating.