I agree. Twenty one days will start to tell the real story...
If the 21 day incubation period (per CDC) is accurate:
Duncan first appeared in the ER on Sept 26. Everyone who was exposed to him at the ER has until October 27th for the all clear
Duncan was transported to the hospital on Sept 28th. Everyone exposed to him during that period (including hospital staff at his arrival) isn't clear until Oct 21st.
Duncan died on Oct 8th. All healthcare workers who attended to him as a patient have to wait until Oct 29th.
It's really much more complicated than this though. For instance, we don't know when the 21-day clock started ticking for the Dallas nurse who is now positive for Ebola, and subsequently when the clock started ticking for everyone who was exposed to her prior to her hospital admittance today.
We also don't know how Duncan's body has been handled, and by whom. Have his remains been cremated yet? If not, there are some additional people (those who handle the remains) for whom the 21-day clock hasn't begun yet.