A simple answer they taught us in the high-school was that you always compare any time period to how long you have lived so far. The older you are the greater the divider.
A given duration of time pales away in comparison to one’s age as he/she grows older.
For example, a 20-year-old sees a decade as half a lifetime. For the 100 year-old it counts as just 10%.
But I do have a memory of my parents leaving me with my grandma once and telling me they would be back in 3 hours. I was 3 or 4 at that time and I remember thinking how mind-bogglingly long these 3 hours felt when they said it.
And, by the way, for me senior year high school seems the longer.
I buy the "you compare each experience to your previous experiences" argument. If new experiences happen at a constant rate, that works out to simple ratios of time, eg. senior year of high school was 1/17 of your life, freshman year of college was 1/18. But new experiences don't always happen at constant rates: some people open up much more in college than in high school. Hence it feels longer.
I suspect that certain experiences occurring at certain periods also have a bearing. I find that some experiences tend to signal the beginning or end of different stages of my life and in some way I'm not fully able to describe, this seems to have some effect on my perception of time as I look back.
But for me, there's definitely something in this comparing any time period to how long you have lived so far.