I've been thinking about the dynamics of having a "slave" 24/7/365. The ideal is that the "slave" signs a contract agreeing to certain considerations and loss of rights. And this assumes that the "slave" can at any time, say "I'm done with this" and walk away. [However, one Mistress (Ingrid Bellemare) took compromising photos of the "slave" at the beginning (with permission of the "slave") and keeps them with the threat of releasing them should the "slave" decide to quit the contract mid-term.]
However, if the "slave" acts in the agreed upon position long enough, doesn't he/she/it become institutionalized. At which point, the "slave" becomes a slave, and there is no more free will. Some evidence are cases of Stockholm Syndrome, Lee Boyd Malvo, countless women who stay with their abusive husbands, ....
I have seen a few references in the literature to having an agreed upon (weekly) time in which the submissive is an equal and can speak freely, but this does not seem to be mentioned enough. Even so, the risk of turning the contracted "slave" into an actual slave would still be present.