400 cigs are allowed to be brought into China regardless of length of stay. 

HugAPanda is correct, each time you enter China you can legally and without paying duty bring in 400 cigarretes (2 cartoons, 20 packs).   If you are over that limit, and get caught, you will be required to pay the excise duty or they may be confiscated as contraband, depending on Official.

Now, I am not recommending to anyone to do this.  It has been my own choice whenever I do it.  But it is based on the fact that I have been visiting or living in China for almost 15 years, and I have passed Customs each time without my suitcases being physically checked.  So, in many occasions I have brought amounts exceeding this limit, and no one has said a thing.  Maybe it is because I do the route into Hong Kong first, then accross the border to Guangzhou, and at border station they are on the look out more for fruits and drugs rather than cigarretes or hard liquor.  If your port of entry is differenmt, conditions may be different too.  Just remember, in China, what works for me may not work for you.  So, YOu decide what you want to do.