There is no specific time limit for how long a cough is lung cancer. Coughing is the main clinical symptom of lung cancer. Patients with lung cancer mostly have irritating dry cough, which is more intense, and sometimes they can cough up purulent sputum and blood in sputum. Therefore, if cough and sputum cannot be relieved by conventional treatment for more than two weeks, it should be taken seriously, especially for patients with high risk factors of lung cancer, and if they cough blood, they should improve chest CT, bronchoscopy and other examinations at an early stage to exclude the presence of lung cancer. Therefore, patients with chronic cough for more than 8 weeks should undergo routine lung cancer screening, especially bronchoscopy, to detect any bronchial lesions. It should also be noted that some lung cancer patients may not have a cough, and the presence of lung cancer can be detected by routine physical examination, especially by CT lung examination, so the length of cough should not be used to determine the presence of lung cancer.