Is it possible to apply for tourist visa that grants me a one month stay in china?

I applied for tourists visa for 2 times already, now I was invited by a friend in Shanghai, the situation is, its always 7days duration for stay in china that granted to me on my previous visa. would it be possible to change it into One Month duration? why is it that it became a trend that only always 7days duration to stay in china?please explain to enlighten me. Thanks a lot