An image forming apparatus is provided. The image forming apparatus includes a reading section configured to read an original document, a printing section configured to perform duplex printing including printing N sheets on first sides thereof and subsequently printing M sheets on second sides thereof when printing an image of the original document read by the reading section, wherein M is equal to or smaller than N, a determination section configured to calculate a preparation time required to generate print data based on a copy setting which affects the preparation time, and configured to determine the value of N, wherein the determination section determines a smaller value as the value of N, as the preparation time is longer, and a control section configured to control the printing section to perform the duplex printing in accordance with the value of N determined by the determination section.