After you've been infected with a pneumonia-causing organism, it takes as little as 1 to 3 days or as long as 7 to 10 days for symptoms to appear.How severe pneumonia is and how long it lasts depend on: 1  Your age and health. 2  Older, sicker people usually have more severe cases.f your pneumonia is caused by a virus or bacteria, you may spread the infection to other people while you are contagious. How long you are contagious depends on what is causing the pneumonia and whether you get treatment. You may be contagious for several days to a week.