Do you mean how long its been up & running or ? Otherwise, right click the taskbar,left click Start Task Manager & click the Performance Tab & it'll show you how long since you (re)booted up.
Of course you can also use the TS search feature top right corner in here.
I mean how long its been up & running but not in a single session but from the very moment it was purchased . I would call it a " total usage time counter" (in hours )
You can type "systeminfo" in to a command prompt and it will give the date the operating system was installed. You can probably work out an average from this.
As far as I know there is no total uptime counter built in to windows, But there is a third party app called winaudit here : http://pxserver.com/WinAudit.htm
Install it and choose the "uptime statistics" option on the left of the screen.
This will however only report from your last install of windows. A fresh install will always reset your counters.