visa-free China - how does it work? How do I know how many days I will get?

Hi, this is the first year my country can enter China visa-free. But how does it work? I'm so used to having all these documents, fly tickets etc. prepared haha. I'm already in China, but my current visa is ending soon. So, I'm thinking of going to HK, then enter China again. But how will I know how many days I get? Do I need to show a return ticket?