What is the optimal duration of therapy in patients with metastatic colon cancer with no evidence of disease?  

First of all, it sounds as if congratulations are in order for your management (to have sustained a patient with mCRC for over 5 years is truly a remarkable accomplishment, although in cases like this the self-pitying oncologist might wonder if they are victims of their own success in determining a ...

I don't think there is a definitive answer to this question. It's very rare for patients to be rendered NED with systemic therapy alone. The body of literature supporting maintenance therapy over chemo-free intervals (very thoroughly outlined by @Mark A. Lewis above) may not apply to this group...