It is impossible to give an exact answer as to how long a lung cancer has been present.

Cancers begin when changes take place in the genes of one or more cells in a particular tissue which cause those cells to divide and multiply in an uncontrolled way.  The different processes that lead to those changes and how long they might take to develop is unclear.

Once the cells start to multiply then they will need to go through many divisions to produce a cancer one centimetre across, which is about the smallest size of lung cancer to show up on an x-ray. This would suggest that from the time the cancer first started until the time it can be diagnosed is months to years. For most of the time the cancer is growing it is too small to cause any symptoms and certainly too small to show up on any x-rays, scans or blood tests. This is why, although your cancer is likely to have been present for years, you have felt quite well until just recently.