Actually, 21 days is only the 99th percentile, not the max. The median onset is at 5.5 days.
Here's a chart from a CDC report, showing the distribution of incubation times across 5,000 patients:
It's interesting that the long tail on the right extends all the way to 25 days. From the link:
The figure above shows the distribution of Ebola virus incubation period, by days of incubation. Data from two sources were used to construct a lognormal probability distribution of being in the incubation state. The mean incubation period derived from this calculation is 6.3 days (standard deviation: 3.31 days), with a median of 5.5 days and a 99th percentile at 21 days.
Yeah, I have posted that graph and noted that we are in the home stretch for Dallas since last contact with Duncan was last Sunday. The people who came into contact can start to breath more easily with each passing day.