Mistress I think you are discussing a slave not a submissive. There is a feel out period between a Dom and sub when a relationship starts I have always noticed that I tend toward being a slave as the relationship grows because I trust owner to make me safe.
Rachel is right there is a big difference between a sub and a slave. Slaves have no rights they only obey. Subs choose to obey, we often live vanilla lives and choose to submit when in session. It's a much more discreet relationship when you are a sub. Although after the feel out period you each know the limits and the sessions don't always have to be sexual. Sometimes they can be just simply making the mistress feel good after a long hard day.
I'd like to add to this. I feel the distinction between a sub and a slave does not apply to how often you do things. The distinction between both is important indeed: A sub is one who is still considered when he obeys, one whose feelings and sometimes decisions still apply to an extent (that can vary between little and a lot). A slave is one who can't complain, can't refuse, and has to fully obey no matter their feelings and emotions and are indeed only allowed to feel.
But, it's easy to imagine a full time sub. It's just one who lives the lifestyle 24/7, but whose feelings and opinion and emotions matter. And likewise, you can also have someone who just lives their life and becomes a full slave only during sessions.
I often considered that difference in how often you do things like this: You can have sessions, and not be a sub or a slave outside of them. You can have a "play partner" (or recurring dom or sub), which is halfway between sessions and a long term thing, and consists of living it sparingly as the situation comes. And then there's full long term, 24/7 style, where you just don't stop being a sub or a slave no matter what. But for all of the above, you can either be one, or the other.
I wont get into the whole slave vs sub distinction since there is no real answer that is fully accurate 100% of the time. Everyone has differing views as to what a sub or slave may be defined as and where those two differ. The important part is that both sides of a relationship clearly communicate in the beginning and establish how they both see things and what limits and expectations are going to be implemented. The actions that cause issue most times are two people with two differing views on what a title is defined as and if not clearly agreed upon this can be disaster to someone emotionally and mentally.