I guess the length of time you kept them in such confinement, would depend on how and what you wanted to accomplish. It is humiliating situation since they are forced in such close proxmity. The rubbing/coseness of intimate parts would create stimulation and shame at the same time. It is a physically demanding position, yet also it could be boring for the victims. Or after a while the muscle ache would numb them.

You could keep them in it for a long time and combine it with punishment. Cold water, over a hot fire, horsehair whips or pleasure --gaps between the bars to accompdate your cock or have a friend join you in beng serviced by the tortured beauties in the cage.

Open for more ideas.