Generally two or three weeks ahead works for getting decent airfares, unless you're travelling at a holiday period.
There can be fairly big seasonal differences in fares, with discounts being greater in the winter than summer.
Also, discounts tend to appear on busy, well-travelled routes. Flights in and out of a small place like Huangshan, for example, often don't get discounted that much.
Thanks - looking at Beijing to xian, Xiang to chengdu and chengdu to Guilin. I'll wait til,3 weeks out.