If you have COPD and smoke cigarettes, how long have you smoked?

I am a student who is learning about COPD and how cigarette smoking may contribute to it. My goal is to ask 100 people who have COPD if they ever smoked cigarettes or not, and how long. Then I will report that data in my science paper. I have access to research, but I am required to gather some data of my own. They do not have to give their name. Any help is appreciated.