How long should it take for an umbilical cord stump to fall off?

I read somewhere anywhere from 7 days to 4 weeks? It'll be 2 weeks tomorrow for my son. And should it greenish underneath? It's becoming looser each day but we don't wiggle it or anything. I looked under it today and it doesn't look gooey or oozing, but it does look greenish. Almost like the actual base of the cord is what has turned green.