OS X (Lion, anyway) doesn’t have a GUI based way to tell how long it’s been running. The server version does but my attempts to look up this information using Activity Monitor and System Information didn’t net anything.
But after searching, I found this link to a terminal tip that gave me what I was looking for. Launch Terminal.app and type in the following:
And the return will be the length of time the computer has been up. Not sure if that’s total CPU time or of that’s since the last reboot, though.