For me - about 10 days.

I didn't notice any fatigue during radiation.  However I got
a little extra sleep and maintained a serious exercise program
before and during radiation.  My ability to exercise was
dramatically reduced (more by the Lupron than the RT I think)
but my energy level stayed more than high enough to handle
ordinary life.

   Alan

I will assume by RT you mean weeks of RT done daily. Seems fatigue hits
different men differently. Apparently some men feel it in the first three
weeks. Those that feel fatigued right in the beginning tend to have more
fatigue as the weeks pass. I have an older friend in his 70's who had his
done and came home every day from about the second week and took a nice long
nap and then he was fine for the rest of the day. (He had also suffered a
minor stroke a few years before the PC) OTOH, my husband really didn't feel
much fatigue from the RT. He took his in the mornings and went straight to
work. He sits at a desk all day so maybe that is different than someone who
is in construction and does hard physical labor. By the end of the third
week my husband was happy to have the excuse to take a nice afternoon nap on
the weekends. And after the third week he began to go to bed about an hour
earlier than normal. At the end of his 5 weeks he was glad it was over and
just about back to normal when about 10 days later he had his brachytherapy.
That is when the fatigue seems to have crept up on him and I think he felt
more fatigued about 2 weeks after that then he had before. He also at that
time picked up a bladder infection and didn't know it and almost was septic
from it when it was caught. So I'm sure that didn't help his overall well
being. I'm not talking about sleepy, or too tired to do anything just a low
draggy feeling. I think the brachytherapy had more effect on his body than
the RT did. And I think it took about 2 years before he really returned to
normal.

Feeling a little tired for a little while is a small price to pay. And I
think the healthier you are going into to it the better you will be able to
handle it.
Bev

As always, YMMV.  However, for me, based on my wife's observations, it took
about 2 months to be my old self again.