Lung cancer risk increases with smoking duration and amount; duration has the most effect: smoking one pack of cigarettes a day for 40 years is more hazardous than smoking two packs a day for 20 years.
Source: Lung cancer risk factors @ Cancer Research UK
Image by Challiyil Eswaramangalath Vipin from Chalakudy, India (cigarette.. like a cigarette in the rain) [CC-BY-SA-2.0], via Wikimedia Commons