On the basis of plane free surface gravitations groundwater flow model and some assumptions similar to ones employing in infiltration and quasistationary problems the time of the necessary lowering of free surface before the building of pit is considered. For the lowering the group of wells along the pit perimeter is used. The wells are located so close to each other that formation a filtration flow by them is equivalent to dewatering by using a narrow vertical slit along the perimeter of the excavation. The time is calculated on the basis of condition that at the beginning of construction the level of ground water within the excavation must be below the design elevation of excavation bottom, and outside it smoothly changing water flow to the slit must be formed. The formulas for the time of lowering from initial natural level before building of pit to finite level equal to the bottom of pit are obtained. The example of calculation is discussed.