I don't notice if the TV is on or off.
You see, I use my TV as my main display for my MacBook. If the main display is not on, the MacBook is asleep, and thus I can't do anything with it.
Of course, I can turn the TV off without putting the computer to sleep, but for me that wouldn't be very useful.
If you do want your Mac Mini to do stuff when the screen is switched off, then getting the screen size is a good start, but getting it to be reliable means the script will have to be on all the time, constantly reading the screen size. Instead, you could make a launchAgent to run on new entries to the windowServer log file. For instance, if I open up the display on my MacBook so that I have two displays running, do a search for 'windowServer' in the Console, so I only see windowServer messages, and then switch off my TV, and then switch it back on again. Among a whole load of other messages, I find these:
23/10/2014 11:38:08.191 WindowServer[110]: Display 0x1b488001 changed state to offline
from reading more of the messages, I can see that display 0x1b488001 is my TV because of this message.
23/10/2014 11:40:05.503 WindowServer[110]: Display 0x1b488001: GL mask 0x2; bounds (0, 0)[1920 x 1080], .........
1920x1080 is my TVs resolution, as opposed to 1366x768 for my MacBook Air's display.
You can use an App like Plisterine or Lingon to make launchAgents. I would suggest starting by locating the log file that contains the windowServer messages; creating a launchAgent to run on new entries; having that launchAgent run a script that will poll the new entries to the file and match your display's hex address with the word 'offline'; and if true, perform the stuff you want the machine to do.
As it happens, I'll be receiving one of the new Mac Minis in a few days. When that happens, I may well find that having the machine do stuff while the display is off is beneficial to me, if that is the case, then I will probably end up writing a system that does exactly that, but for now, it is not beneficial to me.
Stray question:
Is the thing you're trying to schedule, the shutdown time of the machine. i.e., if you turn off the display, the machine also shutsdown?
Because if that's the case, you can do it the other way around. When I'm done for the night. I run a script that sets the shutdown time for the computer, and sets the TVs sleep timer for the same time. That way, they both switch off at the same time.