The validity of China double-entry visa is generally 6 months.

Normally you get 6 months to travel into china counting from the visa issuing date, the length of the period they will allow you to stay all depents on what type of visa you will get, you can always extent your visa once your are in China, but when you have double entry do not extent it during your 1st entry, they will extent your visa but you will loose your second entry!! (just had that experiance with one of my friends)