It's not something to be measured. I perceive it more as an osmotic membrane, permeable, allowing us to flow from past to future. Just as we humans cannot perceive infinity, or eternity, we cannot really perceive an 'instant.' The moment it happens, it's gone. Lots of things don't seem 'real.' An atom is 99.9% empty space, yet matter seems solid. We still don't know what energy is, or gravity. We can only describe how things affect our existence. Existence, consciousness, all are unexplained and perhaps unexplainable.

Science endeavors to answer the question: How.

Philosophy endeavors to answer the question: Why.

I think the two are inseparable if we ever hope to understand the workings of the universe.