It varies depending on the purpose. RT to the prostate and pelvic area has for many years been 37 fractions of 2Gy but more recently, many oncology departments have been giving 19, 20 or 21 fractions of up to 3Gy. The research showed that giving fewer sessions at a higher dose is more effective and has fewer side effects, if the man is fit enough to tolerate it.
So your dad is likely to have between 4 and 7 weeks of treatment.
If RT was being given to distant mets, for example to reduce pain or stabilise a bone, it might only be 1 - 5 sessions.