When I first started working, it was at a dog boarding kennel. I had to hard boil eggs for the dogs one night...
I called my mom and asked "how long do I boil them?". Her answer "when the light comes on they're finished". Had to call a friend's mom for the timing.
My mom always said that electric egg boiler was the best gift she ever received. I have it now.
With new ones, you have to be careful because some don't shut off - or they turn back on after X minutes - so if you're not paying attention, they will overcook the eggs. The best have an automatic shut off and a light that stays on once the cooking is finished until you unplug the unit.