Some experts, including The Master Gardeners of Santa Clara County, do not recognize the efficacy of aspirin on flowers. This group recommends that you avoid using aspirin in vase solutions due to its ineffectiveness in increasing vase life of flowers.
Alternatively, Martha McBurney, a master gardener at the University of Rhode Island, enthusiastically endorses using aspirin on flowers. She not only encourages you to use it in vases, but suggests a spray mixture of aspirin and water for your flowers and plants still growing in the garden.
Dr. Staby's review of research from the 1920's through the present day found consistent inconsistency. Thus, you may find the best option in answering the question of aspirin's effectiveness in extending flower longevity is to research and decide for yourself. At a minimum, experimenting with aspirin will not harm your plant.