The length of your timeline, and subsequently its unit of measurement is tied to how long your composition is set to as well as how zoomed in/out you are to the timeline.
Go to Composition->Composition Settings to see the duration your comp is set to. If it is set to "0:00:00:10", then the comp is only 10 frames long (1/3 of a second) and the timeline will then be measured in frames. If it was set to something like 3:00:00:00, it would be set to 3 hours long and, zoomed all the way out, would be measured in minutes, but zoomed all the way in would be measured in frames.
Hit + or - on your keyboard to zoom in and out of the timeline or slide the "little mountain/big mountain" slider at the bottom to see more or less of the timeline.
My hunch is that your composition is set to be much shorter than you intended.
One way ensure your comp is the correct length when working with footage is to create your comp by dragging the footage onto the "Create a New Comp" button at the bottom of the Project panel where your imported footage is visually stored.