How long can Hong Kong people stay when they travel to the Mainland?

As long as the return card has not expired, you can legally stay in the mainland.

Hong Kong people need a Home Visit Permit to go to the Mainland. As long as the return card has not expired, you can legally stay in the mainland. The card is valid for adults 10 years and minors for 5 years.

The Home Visit Permit for Hong Kong and Macao residents to and from the Mainland, commonly known as Home Visit Permit or Home Visit Permit, is issued by People's Republic of China (PRC) and the Exit-Entry Administration Bureau of the Ministry of Public Security of China for residents of the Hong Kong Special Administrative Region and the Macao Special Administrative Region with China nationality to and from the mainland of China.