How long did the Qin dynasty’s Great Wall construction project last?

The construction project to link existing wall segments into the "10,000-Li Long Wall" under the Qin dynasty began about 214 BCE and lasted for a decade.1

Following the unification of China in 221 BCE, the first emperor, Shihuangdi, ordered Gen. Meng Tian to garrison the northern border and connect the defensive fortifications previously established by the states of Qin, Yan, and Zhao. This massive undertaking involved hundreds of thousands of soldiers and conscripted workers, though the wall fell into disrepair following the collapse of the dynasty after the emperor's death.1