Angie,
I'm one of those who worked for most of the time during treatment. (Shortly before my diagnosis, I had received a significant promotion for which I had worked long and hard, and I felt as though I couldn't be out of the office very long.)
After my neck dissection I was recuperating at home for couple of weeks, but able to do some work there. During radiation I was working partial days at the office (about 5-6 hours a day) for most of the time, but I have to admit fatigue and pain were really setting in. I think I also had another motivation for being at the office during that period -- it seemed that some of my coworkers, who didn't know much about oral cancer, thought the treatment was just slightly more involved than a root canal. I guess I figured that when they saw the radiation burns, the effects on my eating and speech, the weight loss, etc., they might get a better sense of the seriousness of this disease.
It's fairly clear from this series of posts that there's no single answer as to how much or how soon you can expect to work during and after treatment, but your body can tell you a lot about when you're ready.