Find out how long a Mac has been running since last reboot
1. Lunch Finder app.
2. Go to Applications > Utilities
3. Launch Terminal app.
4. Type in “uptime” (without quotes) and press the enter enter key.
5. The time the computer has been up will be displayed. Type exit to quit, then close the window.
Source: http://www.macworld.com/article/1150162/mac_uptime.html