Cocoa,
Welcome to this site -- I'm sure you'll be able to find a great deal of useful information to help your mother during her treatment.
There are many variables that can affect how a person responds to radiation, including: the stage and aggressiveness of the cancer (which will affect the treatment program), and the patient's age and general health at the outset. To emphasize one of Kirk's points, I would echo that I hope your mother is being treated at one of the major cancer centers (there is a list of NCI-designated centers under "Other Resources" on this site). With oral cancer, it is extremely important to be dealing with an experienced, multi-disciplinary head and neck oncology team that can bring all the right resources to bear as early as possible in the process.
Like Kirk, I continued to work during radiation, although my typical work weeks before cancer were in the 50-60 hour range, and during the latter stages of radiation I was probably down to about 30 hours. (Without going into a lot of details, suffice it to say there were certain aspects of my job that I felt I couldn't get away from entirely, even when I was feeling my worst -- which was from about the midpoint of radiation until 3-4 weeks after.) Back when I was treated, chemo was not generally used with radiation, so I didn't have the nausea/constipation issues that tend to be associated with that part of it.
The main side effects were pain/burning in my mouth (with some blistering), occasional infections, loss of taste buds, severe dry mouth and extreme fatigue. All of these things make nutrition a huge challenge -- I'm one of those who didn't have a PEG tube for feeding, but I did have to use "swish and spit" numbing cocktails before every meal once the pain set in. "Meals" generally had to be in liquid or mushy form: Ensure, Instant Breakfast, creamy combinations mixed up in the blender, baby food, cream of wheat -- you get the picture.
The general rule of thumb seems to be that it takes about one month of recovery time for every week of radiation, so your mother should be prepared for the fact that she will not immediately feel better once the treatment ends. Typically, the sense of taste returns gradually over several months and, depending on the type of radiation used, the dry mouth problem may lessen over time.
I'm sure you will also hear from others here who were not able to continue their normal routine due to the severity of the side effects, so realistically you will need to be prepared for a range of possibilities.
Feel free to come back whenever you need to ask more questions or support.
Cathy