1. If you are having L(Tourist visa)

You can extend it to one month by yourself and you can do extension 2 times.

So you will get 2 months for staying in China.

You can ask agency for converting your L visa to F visa 6months if you need to stay more longer. And you just can stay in China max. one year without leaving China.

2. If you are having F(business visa)

You can ask agency for extending F visa to 3/6/12 months F visa.But as the same, you just can stay in China one year max.

3. If you are having X (student visa)

You can convert X visa to F visa if you have a letter which can prove that you had finished your study in China.

Hope it will useful for you~

thanks