To learn how long it's been since you last Shut Down or Restarted your Mac, follow these steps:
1) Launch Terminal. (You can find the Terminal application in the Utilities folder on your Mac.) The easiest way to launch Terminal is to begin typing Terminal in the Spotlight search bar.
2) At the prompt , which will look something like this: Sherlocks-i7-MBP:~ sholmes$ type Uptime after the $ sign and then hit Return on your keyboard.
The first line in the response will tell you how long your Mac has been running.
Sherlocks-i7-MBP:~ sholmes$ Uptime
15:37 up 2 days, 25 mins, 2 users, load averages: 0.09 0.21 0.22
In this case, Sherlock Holmes laptop has been running for 2 days, 25 minutes.
I had occasion to look at an Apple Server this week whose Uptime was 139 days!
3) When you're done, type exit and then hit the Return key again. Finally Quit Terminal from the Terminal Menu.