How often should poinsettias be watered?

You should water it when the soil is dry to the touch.

Water the poinsettia when the pot becomes lightweight or when the soil becomes dry to the touch, about once a week. Water the plant thoroughly in a sink, letting it drain, before putting it back where it was.

I've always let them JUST dry out before rewatering, and only giving enough water so that nothing comes out of the drain holes. That works really well for me. Make sure you keep the plant in the sunlight so no mold develops in the soil/root area.

I water mine every other day but not too much.