Summarize the following:
It's on the top-left corner of the screen. Doing so will display a drop-down menu. This option is near the bottom of the drop-down menu. You'll see this on the left side of the "About This Mac" window. It's on the left side of window. Clicking this heading will open the "Software" overview in the main window here. This option is near the bottom of the list of information in the middle of the page. The number to the right of this heading determines how long your Mac has been on since its last shut-down.

Summary:
Click the Apple icon. Click About This Mac. Click System Report. Click the "Software" heading. Find the "Time since boot" heading.