It’s actually 42 days.
http://www.huffingtonpost.com/manoj-jain-md-mph/ebola-quarantine_b_6009176.html
I’ve read 40 days. Excerpts of this article, saying 5 percent risk after 21 days, and 0 percent risk after 40 days.
“For me, a recent article in the New England Journal of Medicine detailing the first nine months of the 2014 epidemic in West Africa raises concern about the short, often-mentioned 21 post-exposure-day periods in the guidelines. In the journal’s study of 4,507 probable and confirmed cases, “approximately 95 percent of the case patients had symptom onset within 21 days of exposure.” If we do the math, this means that approximately 5 percent or 225 of the Ebola cases in West Africa had symptoms 21 days after exposure, as reported by the patient or caregiver.”
“Charles Haas, professor at Drexel University, published a paper in PLOS Current Outbreaks that can help us delineate the probability of infection, based on the six Ebola epidemics. According to his model, 21 days after exposure the risk of acquiring Ebola is 0.1 percent to 12 percent. The risk reduces to 0.01 percent to 5 percent after 30 days and the risk is virtually zero after 40 days.”