After 16 to 18 days, on average, symptoms can appear. This period between mumps transmission and the start of mumps symptoms is called the incubation period.. In some cases, the incubation period can be as short as 12 days or as long as 25 days.A person is not contagious during the early mumps incubation period.A person is most contagious about three days prior to the onset of the swelling of the salivary glands (called parotitis) to nine days after the onset of symptoms.his period between mumps transmission and the start of mumps symptoms is called the incubation period.. In some cases, the incubation period can be as short as 12 days or as long as 25 days. A person is not contagious during the early mumps incubation period.