I flew to China for 2 weeks with one carryon.
(Came back with rather more luggage... and a kid, with her own luggage. Twice.
)
5 t-shirts, underwear to match, one extra pair of lightweight pants, and I'm good for as long as I need to be there. If I need to "dress up" I throw in a lightweight skirt and a "nicer" t-shirt. Every few days I do laundry in the bathroom sink. Other people I've traveled with think I'm crazy... until they're waiting for their luggage or hauling around heavy bags, and I'm there with my one. But it depends on a fairly informal style of dress, and a willingness to hand-wash clothes as needed.