How long is it supposed to it take for cancer to run its course?

Unpredictable: Cancer is either cured(if it does not recur in 5 years) or it typically relapses /recurrs in 2-3 years.Once it has relapsed it can be kept under control for 2 to5 years depending on the type of cancer. So the course can be as short as 6 months of treatment or chronic recurence for several years or sometimes it can be fatal within a matter of few months.So it varies a lot depending on the type of.

Unpredictable: Precancer and tiny cancers smolder probably for years or decades. The course is famously unpredictable even in cancers with fairly well known typical courses -- and there are literally thousands of subtypes. Acute leukemia and lung cancer can kill within a few days of the onset of symptoms; a low-grade inoperable sarcoma or a tame lymphoid neoplasm can do little for many, many years.