Chest x-rays may take 15 minutes or longer depending on how many different angles your doctor has ordered.
Chest x-rays are painless and emit very small levels of radiation.
Read about a study performed in 2005:
Chest X-Rays Inaccurate for Detecting Lung Cancer
Sources: American College of Physicians Complete Home Medical Guide, Lung Cancer by Lorraine Johnston, and The Yale University School of Medicine Patient's Guide to Medical Tests by Barry L. Zaret, M.D.