Ellen,
my husband's work allows him a good deal of flexibility, is computer related, and does not require a lot of talking. He took the chemo day off through most of CRT, but work either a full or part of a day the other four days. He took the last week of CRT off, and half of the next week, then did part time for a bit. He was able to work a full week about a month after treatment ended - I believe that is an unusually quick recovery.
Based on my single data point of my husband's recovery, my guess is that a) your husband's ability to talk without noisy interuptions due to mucous-related coughing and gagging (if he has these problems) and b) fatigue - will be the main factors in returning to therapy practice. If your husband can schedule breaks between appointments to get his mouth back in order, and rest up a bit, my opinion is that January would be doable. Best of luck with the remaining treatments!