The Duration of the Chinese L visa after enter is 30 days and the maximum is 180 days, if you need extension, just go to the local security bureau（county-level above) to apply for it when you are in China, so you can apply for a visa for atrip to China for 50 days.

may i know where is the local security bureau?
Is there any T&C or certain reason to apply it?

You can call 114 for help to find the local security bureau when you are in China.
If you apply for extension of your stay, you must provide the necessary documents such as your passport, your visa, your reasion for the extension and so on.

den can i extend my visa for 50 days in malaysia?
or only can make it in Chona?

The extension application must be done in China.