My Japanese girlfriend is coming to China on a 3-month single entry visa, but I want to know if the duration of stay for a 3-month single entry is SET at 30 days or if it is possible to get longer, like 60 days, or the entire 3 months(90 day) validity.

Or is it just a question of writing on the application that your intended stay is say, 45 days, and hope that they will give her 60 days duration instead of just 30 days?