How long do yarrow flowers last in a vase?

Yarrow lasts longer than most flowers in a vase. But you need to change the water every few days with water that has some bleach and lemon juice. Make sure you strip the leaves below the water line. Each time you change the water, cut one inch off the stem. As long as you have them in a room that is not too hot, and away from fresh fruit; you can expect it to last over a week or more.