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The attached table is an entrenchment quick reference to give a player an idea of how long it will take to dig in on their chosen defensive line.
Entrenchment parameters can potentially differ even within the same game title so I provided Gettysburg and Overland as baselines. I believe other titles have similar entrenchment parameters.
Trenches are always built as long as the entrenching units dig for enough time. Breastworks are chance based. The column "Average # Turns" is the average number of turns a unit or stack of units of exactly that strength can expect to build a breastworks until they succeed. It is just the mean of the geometric distribution, the formula for which is 1/P.
And yes, a 1 strength unit can potentially build a breastwork that a 1000 strength unit could use. But, using Gettysburg values, it would take that 1 strength unit an average of 5556 turns compared to 6 turns for the 1000 strength unit. In actuality it takes the 1000 strength unit 5.56 turns, but fractions of a turn don't make sense. As you can see from the table, regardless of unit size, on average all units have to put in the same number of "man turns" to build a breastwork.