If I stop chemo, my doctor says my colon cancer will return. When?

Unfortunately, this is a question that even your doctor can't answer conclusively, since no one can make accurate predictions about the timeline for cancer recurrence. Doctors prescribe chemotherapy to catch cancer cells that have moved from the original tumor area to move to other parts of the body. Unlike surgery and radiation treatment, which are localized, chemotherapy is a systemic treatment that will kill cancer cells throughout the body. That's why it's an important part of treatment to prevent recurence.

If side effects such as pain, nausea, or fatigue are what's causing you to consider stopping chemotherapy, talk to your doctor about prescribing additional medications  to help cope with side effects. There are many new treatment options available that can lessen side effects and make it easier to cope with chemotherapy.