I don't take any more for a 30 day trip than I do for a 7 day trip. A couple pairs of pants, a few shirts, a pair of flats or nicer sandals and a good pair of walking shoes, maybe a summery dress and a couple of scarves to dress up an outfit. Of course, underthings and whatever I'm wearing on the plane.
It is weather dependent though, We do cold weather travel sometimes so a couple warmer layering items will added along with a hat and gloves. Appropriate outwear I can usually wear on the plane or I place my jacket inside my suitcase, opened, and pack all of my other items inside the jacket.