How many days does it take to send something from Changzhou, Jiangsu to Chengdu, Sichuan?

It depends on what kind of mail you use. It takes 5-7 days if you use ordinary mail from the post office, and 3 days if you use express mail from the post office. However, express mail is expensive. I suggest you use the express mail service of Shentong or Tong Yuan. The mailing time is 3 days, and the relative price is much cheaper than that of the post office.