I completed radiation treatment 5 weeks ago. I had 1/4 of my tongue removed as well as a neck dissection. My biggest problem now is canker sores under my tongue. How long until these sores usually go away? Does anyone have any advice on how to deal with them now?

These are most likely not canker sores (herpes simplex), but residual mucositis from the radiation treatments. Mine finally resolved about 8 weeks after the end of radiation. There is little that you can do to speed up their resolution. It is mostly about dealing with the discomfort until your body can repair itself. For me this meant pain meds and constant swishing of my mouth with baking soda and water. It just takes time....