How much lung cancer can you have and not know it?

Depends: A small tumor strategically located can give you symptoms. Extensive disease even metastatic to many sites and organs can be asymptomatic.

Depends on type: Small cell cancers can be widely metastatic and kill with the primary tumor being barely detectable. Squamous cell carcinomas may grow to large size, e.g., size of an egg, before causing any symptoms. Bronchoalvelolar carcinomas may not produce a mass at all but just infiltrate the alveoli.