How many days will I get if I extend my L visa?

Hi,
I'm from Vietnam and since last year, Vietnamese citizens can only get 15 days for L type. Somebody knows if I try to extend in Beijing, how many days can I take? 15 or 30?

Besides, if here in Hanoi, Vietnam, I am told that now it's hard for Vietnamese to extend L type, may it be easier if I try making it in Beijing? I'm very concerned coz it's very to schedule with just 15 days in China. Thanks