Question: How long was the city where the Yongle Emperor greeted the leader that the edict was addressed to, the capital city of Yangzhou?

Knowledge 1: Sino-Tibetan relations during the Ming dynasty
The Information Office of the State Council of the PRC preserves an edict of the Zhengtong Emperor (r. 1435–1449) addressed to the Karmapa in 1445, written after the latter's agent had brought holy relics to the Ming court. Zhengtong had the following message delivered to the Great Treasure Prince of Dharma, the Karmapa: [SEP] Knowledge 2: Nanjing
Zhu Yousong, however, fared a lot worse than his ancestor Zhu Yuanzhang three centuries earlier. Beset by factional conflicts, his regime could not offer effective resistance to Qing forces, when the Qing army, led by the Manchu prince Dodo approached Jiangnan the next spring. Days after Yangzhou fell to the Manchus in late May 1645, the Hongguang Emperor fled Nanjing, and the imperial Ming Palace was looted by local residents. On June 6, Dodo's troops approached Nanjing, and the commander of the city's garrison, Zhao the Earl of Xincheng, promptly surrendered the city to them. The Manchus soon ordered all male residents of the city to shave their heads in the Manchu queue way. They requisitioned a large section of the city for the bannermen's cantonment, and destroyed the former imperial Ming Palace, but otherwise the city was spared the mass murders and destruction that befell Yangzhou. [SEP] Knowledge 3: Sino-Tibetan relations during the Ming dynasty
During his travels beginning in 1403, Deshin Shekpa was induced by further exhortations by the Ming court to visit Nanjing by April 10, 1407. Norbu writes that the Yongle Emperor, following the tradition of Mongol emperors and their reverence for the Sakya lamas, showed an enormous amount of deference towards Deshin Shekpa. The Yongle Emperor came out of the palace in Nanjing to greet the Karmapa and did not require him to kowtow like a tributary vassal. According to Karma Thinley, the emperor gave the Karmapa the place of honor at his left, and on a higher throne than his own. Rossabi and others describe a similar arrangement made by Kublai Khan and the Sakya Phagpa lama, writing that Kublai would "sit on a lower platform than the Tibetan cleric" when receiving religious instructions from him.
Knowledge 1: Qing dynasty
On 12 February 1912, after rounds of negotiations, Longyu issued an imperial edict bringing about the abdication of the child emperor Puyi. This brought an end to over 2,000 years of Imperial China and began an extended period of instability of warlord factionalism. The unorganized political and economic systems combined with a widespread criticism of Chinese culture led to questioning and doubt about the future. In the 1930s, the Empire of Japan invaded Northeast China and founded Manchukuo in 1932, with Puyi, as the emperor. After the invasion by the Soviet Union, Manchukuo collapsed in 1945. [SEP] Knowledge 2: Nanjing
Zhu Yousong, however, fared a lot worse than his ancestor Zhu Yuanzhang three centuries earlier. Beset by factional conflicts, his regime could not offer effective resistance to Qing forces, when the Qing army, led by the Manchu prince Dodo approached Jiangnan the next spring. Days after Yangzhou fell to the Manchus in late May 1645, the Hongguang Emperor fled Nanjing, and the imperial Ming Palace was looted by local residents. On June 6, Dodo's troops approached Nanjing, and the commander of the city's garrison, Zhao the Earl of Xincheng, promptly surrendered the city to them. The Manchus soon ordered all male residents of the city to shave their heads in the Manchu queue way. They requisitioned a large section of the city for the bannermen's cantonment, and destroyed the former imperial Ming Palace, but otherwise the city was spared the mass murders and destruction that befell Yangzhou. [SEP] Knowledge 3: Sino-Tibetan relations during the Ming dynasty
In his usurpation of the throne from the Jianwen Emperor (r. 1398–1402), the Yongle Emperor was aided by the Buddhist monk Yao Guangxiao, and like his father, the Hongwu Emperor, the Yongle Emperor was "well-disposed towards Buddhism", claims Rossabi. On March 10, 1403, the Yongle Emperor invited Deshin Shekpa, 5th Karmapa Lama (1384–1415), to his court, even though the fourth Karmapa had rejected the invitation of the Hongwu Emperor. A Tibetan translation in the 16th century preserves the letter of the Yongle Emperor, which the Association for Asian Studies notes is polite and complimentary towards the Karmapa. The letter of invitation reads,
Knowledge 1: Sino-Tibetan relations during the Ming dynasty
The Information Office of the State Council of the PRC preserves an edict of the Zhengtong Emperor (r. 1435–1449) addressed to the Karmapa in 1445, written after the latter's agent had brought holy relics to the Ming court. Zhengtong had the following message delivered to the Great Treasure Prince of Dharma, the Karmapa: [SEP] Knowledge 2: Sino-Tibetan relations during the Ming dynasty
In order to seek out the Karmapa, the Yongle Emperor dispatched his eunuch Hou Xian and the Buddhist monk Zhi Guang (d. 1435) to Tibet. Traveling to Lhasa either through Qinghai or via the Silk Road to Khotan, Hou Xian and Zhi Guang did not return to Nanjing until 1407. [SEP] Knowledge 3: Nanjing
Archaeological discovery shows that "Nanjing Man" lived in more than 500 thousand years ago. Zun, a kind of wine vessel, was found to exist in Beiyinyangying culture of Nanjing in about 5000 years ago. In the late period of Shang dynasty, Taibo of Zhou came to Jiangnan and established Wu state, and the first stop is in Nanjing area according to some historians based on discoveries in Taowu and Hushu culture. According to legend,[which?] Fuchai, King of the State of Wu, founded a fort named Yecheng (冶城) in today's Nanjing area in 495 BC. Later in 473 BC, the State of Yue conquered Wu and constructed the fort of Yuecheng (越城) on the outskirts of the present-day Zhonghua Gate. In 333 BC, after eliminating the State of Yue, the State of Chu built Jinling Yi (金陵邑) in the western part of present-day Nanjing. It was renamed Moling (秣陵) during reign of Qin Shi Huang. Since then, the city experienced destruction and renewal many times.[citation needed] The area was successively part of Kuaiji, Zhang and Danyang prefectures in Qin and Han dynasty, and part of Yangzhou region which was established as the nation's 13 supervisory and administrative regions in the 5th year of Yuanfeng in Han dynasty (106 BC). Nanjing was later the capital city of Danyang Prefecture, and had been the capital city of Yangzhou for about 400 years from late Han to early Tang.

Answer: about 400 years