How close to expiration can I enter China?

I think I have my answer.

From: the faq page of the Houston consulate
5. I'm going to China on June 1, and my current one will expire on June 10 with its during of stay 30 days. Can I still stay in China for 30 days?
Yes. Here are some Explanations of Number of Entries, Validation and Duration of Stay.
Validation is a period of time, during which entr permit is good for the holder to enter. Validation starts from the date of issuance and the expiration date is shown as ENTER BEFORE on the entry permit.
Number of Entries designates how many times one can be used to enter within its validation. Please note each time you travel from Hong Kong or Macao to Mainland, one entry is counted. Single, double and multiple entry means within the validation, you can enter once, twice and unlimited times respectively.
Duration of Stay sets the maximum days of stay counting from the date of entry. Example: Mr. A has a single entry one showing ENTER BEFORE Dec-20-2003 with duration of stay 30 days. If Mr. A enters PRC on Dec-18-2003, he can still stay up to 30 days.