According to the Centers for Disease Control and Prevention, Ebola has an incubation period of 2 - 21 days. That is the time interval from infection with the virus to onset of symptoms. During this time, viral genomes are replicating and the host is responding, producing cytokines such as interferon that can have global effects, leading to the classical symptoms of an acute infection
However, studies have shown that this number may not in fact be correct.
Longer Incubation Periods
Groups such as The National Institute of Health, the New England Journal of Medicine, and PLOS Currents have all show than Ebola can incubate for longer than the previously state CDC and WHO claims of 21 days.
The National Institute of Health
The National Institute of Health analyzed data collected during the Ebola outbreak (subtype Zaire) in Kikwit, Democratic Republic of the Congo, in 1995 using maximum likelihood inference and assuming a log-normally distributed incubation period. The mean incubation period was estimated to be 12.7 days (standard deviation 4.31 days), indicating that about 4.1% of patients may have incubation periods longer than 21 days. The NHI concluded that if the risk of new cases is to be reduced to 1% then 25 days should be used when investigating the source of an outbreak, when determining the duration of surveillance for contacts, and when declaring the end of an outbreak.
The World Health Organization
Recent studies conducted in West Africa have demonstrated that 95% of confirmed cases have an incubation period in the range of 1 to 21 days; 98% have an incubation period that falls within the 1 to 42 day interval. WHO is therefore confident that detection of no new cases, with active surveillance in place, throughout this 42-day period means that an Ebola outbreak is indeed over.
The New England Journal of Medicine
The mean incubation period was 11.4 days. Time between Exposure and Disease Onset, and did not vary by country. Approximately 95% of the case patients had symptom onset within 21 days after exposure, which is the recommended period for follow-up of contacts. The estimated mean (±SD) serial interval was 15.3±9.3 days, which is the same as the estimated mean generation time (see Supplementary Appendix 1).
The measured duration of the incubation period, and its variation, imply that the advice to follow case contacts for 21 days is appropriate. To curtail transmission in the community, the period from symptom onset to hospitalization (a mean of 5 days but a maximum of >40 days)
PLSO Currents has mapped out as a possible time line for incubation base off of modeling. 
The current operative guidance on quarantine periods for Ebola (Zaire) virus is 21 days, based on WHO assessment that the incubation period is 2–21 days. A current review of previous outbreaks cites the same range . The precise origin of this assessment is unclear, however it is possibly based on the study of the either the 1976 Zaire outbreak11 or 2000 Uganda outbreak both of which reported (without detailed analysis) a maximum observed incubation time of 21 days.
At a 21 day quarantine period, using the data sets other than the Congo analysis of Chowell, three Congo data analyses, the probability of exceedance is between 1.9 and 12%. In other words from 0.1 to 12% of the time, an individual case will have a greater incubation time than 21 days. The 0.1% stems from the 1976 Zaire outbreak, which has the fewest cases analyzed than any of the others in Figure 2. If the estimated incubation time distributions for the Congo outbreak (Figure 2) are used, this would suggest a quarantine time of as high as 31 days. 
Quarantine of Nigerian Soldiers
About 850 Nigerian soldiers on peacekeeping mission in Liberia are to be quarantined for 28 days as soon as they return to the country in March 2015.
There are two battalions of the Nigerian Army in Liberia.
The Army personnel were scheduled to return to the country in January but that their arrival would be delayed till March 2015.
The Head of the Nigerian Army Medical Corps, Major-General Obashina Ogunbiyi, had said some Nigerian soldiers had been quarantined in Liberia following the death of a Sudanese who went to their camp to pray with them.