If memory serves, the incubation period is 7-12 days. Having them on pasture and moving them frequently should prevent any new ones from getting sick and some who are sick may recover. A friend went through this issue a couple of months ago--but on a much larger scale. He lost a lot of birds...
Is there a way to find out what strain of CX you've got? I would definitely get them on pasture sooner than later. What you're describing sounds a lot like Viral tenosynovitis I think it would probably also be good to lower your protein to give their bones more time to develop. Add a pound...