It may not be related to radiation treatment. Check your blood to make sure you are not anemic or heart damage from chemotherapy such as from adriiamycin.
Radiation fatigue is usually happen during treatment if you receive treatment to large area.It is not common to occur after treatment except underlying disease.

Everyone is different, but about a month seems to be the most common. If longer, you might consider consulting your oncology team.
Thank you and all the best.