melissa.
each single blossom may last a day or three. but if you had a whole clump with many stalks, then your blooming irises (as a whole clump) would give you a flower show for many weeks depending on how many rhizomes and their growing conditions. each flower only lasts a few days.
if they are crowded as robin mentioned the flowers will diminish in size and production.
hope this helps a little, and didn't confuse you more?
