multiple visits

I used to live in China for two years and currently have a bunch of friends there because of it. I have been back in the states for about a year now and I am planning on making some trips to different cities in PRC to visit friends. Each month I get about a week off from work so will be going back and forth just for visiting over the next year or two. Do I need a new entry permit each time? or for a multiple entry visa how many times can I and how long is it good for?