As long as its plugged in, it can stay on forever in standby, though you might want to set it to hibernate after a long while of non-use. On battery, make sure you have it set for battery performance, lower your screen brightness, set the screen to go off after a few minutes of non use, and disable any Wireless or USB or Firewire peripherils you're not actually using, and you'll get the maximum battery performance. All my machines, laptops and towers alike, run 24/7/365, unless I'm working on them. One of 'em is coming up on its 6th birthday. An occasional reboot is a good idea for always-on machines, but I think there's no real reason to shut one down completely as long as it has external power and you aren't poking around inside its case or instaslling a new peripheral that ought to be first connected on bootup. Let it shut down its screen on its own after a while, and "go to sleep" (Standby). About all I ever use "Hibernate" for is changing a laptop's battery in the field without shutting it down and loosing track of what it was doing. My laptops are all set to Hibernate if battery power falls to 8% ... just in case I don't notice ... which happens sometimes. Actually, it happens often enough that I decided on the 8% thing