Not everyone is 24/7.
When that's the case, someone has to initiate. I would presume that in a "proper" D/s relationship, the sub may initiate and the dom may agree... or the dom may initiate and the sub must agree. But even then, even if 24/7, the sub has options depending on vanilla obligations, state of physical or mental health (aka mood) and a host of other potential mitigators.