According to American Thoracic Society (ATS) guidelines, nosocomial pneumonia (also known as hospital-acquired pneumonia or health care–associated pneumonia) is defined as pneumonia that occurs more than 48 hours after hospital admission but that was not incubating at the time of admission. Ventilator associated pneumonia is defined as pneumonia that occurs after 48-72 hours of endotracheal intubation.
ACCORDING TO MEDSCAPE.
And "wikipedia" says same within 48 hrs of admission but also adds that :
within 30 days after discharge .........