How long can someone live with untreated colon or stomach cancer?

Depends: Survival is dependent upon what stage the patient is in at time of diagnosis. If the patient has stage 4 (metastatic disease) at presentation and has no treatment statistically they may live 6-12 months (some longer, some less). The earlier the stage (1, 2, 3) the longer the survival.

Many months: These are widely different cancers with differing life expectancies. Life expectancy also depends upon the stage of the cancer at time of diagnosis. At advanced stages, life expectancy is short and measured in weeks or months. In early stages, life expectancy could reach as long as a year or more. More information regarding the type and stage would be needed to give you the best answer.