After establishing the Ming Dynasty, Zhu Yuanzhang began to consolidate the authority of the royal court. He assigned territories to the members of the royal family and stationed them across the empire. These members of the royal family did not have the administrative power over their territory, but they were entitled to a personal army that ranges from 3,000 to 19,000 men. Royal members that were stationed in the northern frontier were entitled to even larger forces. For instance, the Prince of Ning was said to have an army of over 80,000 men. The original crown prince Zhu Biao died at 36 in 1392, and his son Zhu Yunwen was made the crown prince. Zhu Yunwen was the nephew of the territorial princes, and he felt threatened by their military power. In May 1398, Zhu Yunwen ascended to the throne and became the Jianwen Emperor after the death of Zhu Yuanzhang. The princes were ordered to stay in their respective territories as the new emperor began planning for the reduction of their military power with his close associates Qi Tai and Huang Zicheng.
Question: How many years was Zhu Yunwen a crown prince?
Answer: 6