How many hours since I unplugged my Mac?

macos mac battery charging

On Mavericks take a look in the Activity Monitor's Energy sections bottom.

enter image description here

Related

How do you keep a window in all spaces?
Is there a way to track my look ups in the OSX dictionary?
ssh-add every time I want to use SSH to login to my remote server
Bulk remove provisioning profiles on iPhone
AppleScript to show package contents of hidden original of alias
Terminal Equivalent of Finder Sort Order
Where does the Apple app Contacts store its files in Mavericks?
Forward or Bcc a copy of all outgoing mail
Do I really need to replace my MacBook Pro?
Enable random message at login screen
How do I get rid of errors for disconnected networked drives?
Mac wont boot and fan runs high