We did the China/Yangste River Tour in May of this year. We brought enough clothes for all 21 days ie. 21 t's, 21 underwear, 21 sox, a few pairs of pants, a few pairs of shorts. Two pairs of pants had zip off legs for warmer temps too. We threw away our sox and underwear each day and left the t-shirts at each hotel, folded and in a pile. Our hotel cleaning person was very thankful for the shirts!
Also, as a side note....if you want to do something nice for your main guide as well as the local guides and bus drivers, think about bringing each of them some small gift. We brought baseball hats that had either USA or NY on them. They were bought at a local flea market for a very low price and, yes, we had to cut out the tags, "made in China." All the guides and drivers were MOST appreciative!!
This trip was AWESOME!!!