When to apply for visa?

In May 2017 my wife and I planned a trip to Japan and China with a possible visit to another Asian country in the middle. I have already booked air travel to Japan and return air travel from Bejing to USA. What is the best time to start applying for the Chinese tourist L type? Do I need to submit copy of airline reservation entering China (do not have yet) or just the airline reservation departing Bejing to USA (already have).