How many years is the patent period of corn seeds?

According to Article 34 of China's Regulations on the Protection of New Plant Varieties, the protection period of variety rights of lianas, trees, fruit trees and ornamental trees is 20 years, and other plants are 15 years from the date of authorization. That is, corn seeds 15 years old. But crops are updated quickly, and a variety may not be used for 15 years.