I come from the opposite direction: I'm not so sure humans are all that conscious. My personal theory goes something like:

* Consciousness is a side-effect of the brain running "what-if" scenarios that include anticipating it's own actions. Self-awareness. This definition allows for non-human consciousness, but also doesn't take a stance on it, as it also allows for a range of awareness, rather than a binary state. (that said, I'll talk in binary terms for simplicity)

* Consciousness is calorically expensive - the brain shuts it down when it isn't needed, because it stops adding value then in terms of survival.

* Side effect: The primary job of consciousness is to figure out how to turn the unexpected into the routine, which in turn shuts down consciousness.

Result: We spend a lot of time NOT self-aware, then back-fill that we must always be self-aware when we consider that time (because we are self-aware during that consideration). Impossible to self-measure, but my personal guess is that we spend somewhere between a high number of minutes to low number of hours "sentient" each day.

Note: I'm using Self-aware/consciousness rather broadly here. Even if it's illusion, there's a state of having that illusion (or however to phrase that) and a state of not being aware that you aren't even considering...anything. (Dang it is hard to describe that without using language that assumes awareness).