So I'm the same as you, no symptoms except for one night. I started coughing up blood like I mentioned above, and the reason for that was that I ended up getting Klebliessa pneumonia with the Mac. It took bronchoscopy to diagnose both. So as I was getting more and more sick from the pneumonia, in my head and in my doctor's perspective because they saw a mass on my lung (and found cancer on my intestine) I thought the end was near! You hear about people finding out they have lung cancer and then getting really sick quickly. That's what I thought was happening. As soon as they found that mass from the CT scan I gradually started coughing more and I could hear myself wheezing and coughing a bunch of crap up. That was all from the pneumonia though, and once they put me on the antibiotics it started clearing the pneumonia up and that's about the time they figured out I have Mac because it takes so long to grow in the petri dish. It takes a weeks for them to figure that out!!
Regarding how long you've had it, I was going to tell you that my cavity was pretty large. I'm not sure if you have a cavity or how big of an area is affected, but my cavity was around 3 in and my doctor looked at a CT scan I had done in 2011 and thought that I may have started developing this back then. The reason that he thought that was there was many small little nodules all in that area that I ended up having the cavity. Infectious disease doctors see this often enough where they can determine to some degree how long you've had it. He said I've at least had this 5 years, but he said he wouldn't be surprised if I've had it 10 + the nodules right in the area that I got the cavity is very telling!!!
You probably had this for a while. All depending on how large of an area of your lung is being affected. It's very slow growing. Have you had any other CT scans done in recent years they could compare? You could ask your doctor what they believe. It is slow growing so a large area would take years, small area months to a couple years!!