can i apply visiting visa early( October )? can i get one for 6 month ?

It's normally advised to apply for the visa around one month in advance, but you can apply for it in October if you will go in December, for it is usually valid for 3 months from the date of the issue. Just make sure that your it is still valid when you enter China.

The general documents are your passport, application form. one passport-size photo, two-way air ticket, hotels booking and financial sranding of enough funds to cover the stay.
For business, you also have to provide invitation letter from a partner or cooperator in China and confirmation of invitation from the local foreign affairs office.