When I was in grade school, time was measured by school years. When I was in college, I measured time in semesters. When I started working full time, I felt lost without milestones to measure by. I couldn't remember if something happened one year ago or two.
Eventually, I adjusted to using seasons, calendar years, and life events to measure large spans of time. It's not perfect because major life events just aren't that common and because seasons all tend to look the same after a while, varied by how the garden did that year or how much snow we got.
When I think of time, I think of a long spiraling scroll with previous years to the left and the future off to the right. The further to the left something is on the scroll, the longer ago it happened. As a visualization, this works fine but it lacks the mental highlights of what happened when. I couldn't tell you what I was doing in 2006, for example, without some other milestone to anchor the year to.
At some point, I got the idea to try to make this spiral of time more concrete. I have drawn up plans for wheels, ribbons, paper chains, picture collages, computer programs, seasonal clocks, and large-scale calendars all in an attempt to put perspective on time. None of these have felt quite right, so I'm still looking.
It seems that, as I get older, projects are longer term. In school I could focus on the semester or the school year. Now my plans stretch into the future on the order of years, not months. As my projects get longer, a sense of perspective on time seems even more important.