orangeLearner:
I can't use an LED because LEDs are not low-power.
It's possible to get very small LEDs that don't use much power. How much energy they use would depend how long you left them on after your time had elapsed.
I suppose that an LCD would use less power. Watches routinely run them for years on a small battery. I've no idea how to drive an LCD display but I imagine you could find out.