How long will it take for hair to grow back after receiving radiation therapy on the head?

Yes. Radiation or chemotherapy drugs are toxic to cells that are rapidly dividing. This means they specifically target cancer cells, but they also kill healthy cells that grow quickly. This includes hair cells (causing baldness/alopecia) and stomach cells (causing nausea or mucositis). Once the chemo or radiation is stopped, the side effects go away and hair should grow back.
Depends on the dose. The scalp hair follicles generally regrow hair if the dose they received was less than 45 gy. Your radiation oncologist can determine the actual dose delivered to the scalp by reviewing the plan. If the hair is going to grow back, it usually starts within a few weeks to a couple of months after treatment.