Spring time, when they start to make drone brood.
I bought a couple of frames years ago and gave it a try. One of the frames got filled with honey. Another problem is you put a frames in your brood box, and you have to dig it back out to freeze it. It gets hard when there is a stack of honey supers on top. So I quit doing it real quick.
One frame in the top brood box. Once the queen fills it with drone and they get capped, stick it in the freezer for a day to kill the brood and any mites that are under the caps. Stick it back and the bees will clean out the dead and start it over again. Keep the drone population down a little and remove some reproducing mites.