My dad is 90. was diagnosed with colon cancer last june. it was stage 3 and still localized. how long would he be able to live? he wants no treatment.

Hard to know: Each person's survival after a cancer diagnosis is highly variable. Statistically on the cancer.Org site, the ranges of different stage 3 colon cancers (3a, 3b, or 3c) can range from 20% to 70% over 5 years. Your father's own physician would be much more able to discuss this information with you and him.

Varies alot: I assume your father had the cancer removed surgically. If so, he might have been cured of colon cancer. At the age of 90, and depending on his overall health, chemotherapy would not likely add much to the chance of being cured. I would estimate that he has about a 50% chance of not having a recurrence of his colon cancer, and dying of other causes or "old age".