Is there a time frame between surgery and radiation, after which, the benefit of adjuvant radiation is lost and no longer indicated for head and neck cancers?  

Unfortunately, there are some patients that have post-operative delays. I don't know that the value of adjuvant RT "is lost" beyond 6-8 weeks, but we know the oncologic outcome can be worse. I would consider getting a high quality reassessment imaging either with CT, MRI and/or PET/CT of head and ne...

There is no such well-defined time frame for radiation therapy in an adjuvant setting beyond which the benefit of RT is definitely lost. In general, the sooner the RT is started , the better it is. If a patient is 3-4 months out from surgery a discussion regarding potential benefits vs risks can be ...

Probably 6 weeks; 8 at the most. The higher the risk (positive margins, ECE), the shorter the window.