Great post. I've done some personal reflecting on this very subject and even tested some of my own theories.

I hypothesize that the subjective length of time is determined by the number of new experiences over that period of time. The reason for this being that a new experience, or memorable experience adds a marker in our brains for later recollection.

When reviewing those markers over a period of time, we mentally count and recollect them. The more we have, the longer the perceived length of time.

I personally tested this theory when I lived in Taiwan for a year. While it was only a year in length, it felt much longer. The reason for this was clear. Everything was new: the language, the smells, the buildings, signs, food - everything.