Yep, China domestic is different from the rest of the world for flight pricing - I'd say never book more than 2 weeks in advance, I'd usually book about 7-5 days out. I once booked 10 days out but the price dropped so much that it was worth paying the cancellation fee and rebooking at the lower fare.

Also, book through Trip.com. A lot of the other western search engines never find the cheapest fares, even the international versions of the airlines' own websites. Trip.com service is generally very good.