The simplest way to watch how long it has been turned on for your Mac was booted is to use computer Profiler. From anyplace in Mac OS X, press Command + Spacebar key button and type System Information followed by the Return button to launch the program. Previous or old version of OS X it was known as System Profiler. Select the application from the side menu list and find Time since boot to watch the time it has been since the Mac has been turned on.
A command approach is also realizable. Simply launch the Terminal and type the ‘uptime’ command into the command line to detect precisely how long the system is working. Theoretically, people could leave a Mac system turned on all the time, and also leave it turned on forever. Yes, system will use the power, and people should take that into consideration.
In my case, I personally never turn off my Mac computer until I am going to the outside and not using the Mac system a couple of days, otherwise I just left it on and never turn it off at all. Certain, I reboot to install the application and what not, but that is about it. I set it to sleep if I am taking it somewhere, or want to power it down without shutdown. I essentially never turn off my system, and I have never had issue with it.
Now the question is how long people can really leave a Mac system turned on? That is very hard to say, if people just let a Mac sit and do nothing for long time like a year, yet some hardware components of the system may fail, but this could take a very long time to occur. In fact, many Mac servers routinely accomplish very high uptimes well over a year of being turned on without a reboot.