Once it starts, how long does it take for lung cancer to grow?

Depends: There ars several different cancerous cell types. Some are fast growing and aggressive, while others are slow growing and less aggressive. There are widespread facts on each type available, but a diagnosis from biopsy is needed to answer your question accurately!

Variable: There are different cell types and growth rates( called doubling times)! the small cell, undifferentiated as an example is very fast and most folks die within a year(?Or 2). Some of the squamous cancers are slower and may be quite large and asymptomatic even when large.