Sleep-Wake – How to Prevent Mac Sleep from Command Line

There's no real reason that I can think of not to leave it on all of the time, but you can easily turn it on and off by loading or unloading the launchd plist:

This will turn it on:

launchctl load ~/Library/LaunchAgents/com.ecamm.printopia.plist

This will turn it off:

launchctl unload ~/Library/LaunchAgents/com.ecamm.printopia.plist

Or you could just run the server directly:

/Library/PreferencePanes/Printopia.prefPane/Contents/MacOS/Printopia Server.app/Contents/MacOS/Printopia Server

(that's all one line)

Though not directly answering your question this line will give you the time in seconds since the last user interaction which comes from what OS X considers to be an idle user. (independently of screen-savers or black displays.)

echo $((`ioreg -c IOHIDSystem | sed -e '/HIDIdleTime/ !{ d' -e 't' -e '}' -e 's/.* = //g' -e 'q'` / 1000000000))