To work out how long it will take to complete a task, you can use the 80/20 Rule. It states that 80% of your outcome will be from 20% of your input. If it took 8 men 10 hours to build a wall, then it could take 4 men 4 hours to build the same wall.
If you had eight men working ten hours a day, five days a week, it would take them five weeks to build a wall. But if you had four men working eight hours a day, five days a week, it would take them only two weeks to build the same wall.