Depends on dose/type
Depending on what disease is being treated, radiation is given in between 2 weeks to 7 weeks in standard fractionations.
Near 7 weeks of radiation we expect no further hair growth. About 2-4 weeks, we can expect recovery of hair growth. I am assuming you are talking about x-ray/therapeutic radiation for cancer
. There are other types of radiation that the above answer may or may not hold true.