For what length of time can you have a basal cell skin cancer on your face without knowing it?

Years sometimes: Basal cells start small and some can be very slow growing. Some people get used to them and don't worry unless they bleed or people start asking about them. They know they are there, they just ignore them.

One variety hides: Most forms of skin cancer grow outward an are noticed. One form, morpheaform looks like a flat white freckle and can go unnoticed. It is also the most likely to recur after surgery.