Visa expiration time?

I'm a UK citizen living in Tokyo on a one-year working visa, 4 months of which have passed.

I plan to visit Shanghai in Dec, and have been told that I will need at least 6 months left on my Japanese visa in order to be granted a Chinese visitors visa.

Is it possible to get the visa now and then travel in Dec, or is there a certain time limit between the visa being issued and the date of entry into China?

Thanks in advance!