How long might one have basal cell skin cancer on his face without knowing it?

Basal cell cancer: Basal cell cancer, the second most common skin cancer, is very slow growing. Depending on the location, on how perceptive and how educated one is about these, it could be there for 1-2 years before drawing attention.

Greatly varies: It may take years before someone may become aware of a skin cancer. There are slow growing cancers that grow underneath the surface and are more difficult to detect. That's why someone may have a very large skin cancer (growing deep but not appearing on the surface) and not be aware until it's grown very big and appears on the surface. A skin cancer may pop up suddenly and grow fast within days.

Varies: Perhaps for a long time.