I first noticed this as a young child. The duration of grade school semesters initially seemed like eons, as the years past, the wait for spring break seemed to be less and less. Spring break also seemed shorter. If you go on a road trip to an unfamiliar place, you'll experience the same phenomenon. On the way to the destination, you are much more observant. You're witnessing new scenary (especially if its a different region from where you live), your also anticipating reaching the destination, and focused on navigation. On the return trip, you've already experienced the environment and it feels as though your journey isn't as long. Subsequent trips are exponentially faster as you become more familiar with the things previously mentioned. There is an easy way to slow your preception of time though, simply focus on a clock and watch the seconds tick by. Ten minutes will feel like an hour. With total sensory deprivation, your experience of the passage of time will slow at a far greater magnitude.