Would 12 hours of light be too stressful?

I doubt the light on that long would bother the fish but it might lead to algae issues. External lighting other than what comes from ambient sources (windows) is more for the enjoyment of the viewer (you) than the needs of the inhabitants. Why not just put the lights on when you are there? Avoid both the algae and the expense of electricity and wear on the equipment to no purpose.

I'm not sure about GBRs, but my Bolivian Rams hate anything brighter than ambient room light, and even that's too much sometimes. I have a very small, dim light that I rarely turn on because they'll all huddle at the opposite end like I dropped in a piranha or something.

You can get an indoor use timer at a hardware store for less than $10. Then you won't have to worry about turning the lights on and off yourself. It isn't an issue about the fish, you just shouldn't run the lights that long. Go with 8-10 hours max. That's true even if you're growing live plants. If you don't have live plants competing with algae at all, you definitely won't want the excessive algae growth that much light will support.

fish will find food in the dark. they use more than just their eyes to find food. but you don't need to feed them twice a day, once is enough. why not get a light timer and have it turn on about an hour before you get home from work, and then you'll have the whole evening to enjoy your tank and feed them then, then the lights can go off automatically after 8 to 10 hours, once you are in bed. then just ignore the tank in the morning when you are leaving for work.

I always just have my timers set for on when I am around the tank the most. Generally that is later in the day so the tank will also get some ambient light for the fish.

Has anyone had poor luck with the mechanical timers? I have a fluorescent light on one, and it flickers like crazy when the timer is turning to 'off'. I'm worried that it's going to blow up or something...

Also, has anyone had comparisons between different digital timers? I have one of these, and am finding it is quite nifty compared to the old-school timers, such as by having different schedules for different days.

Well (call me evil) I have had my lights on for 24 hours at a time, 24 on, 24 off, and the fish don't seem to mind. The algae loves it a little too much though...