If the rhizomes start to grow over the top of each other, they will need to be thinned. Thinning some out every year allows you to maintain a clump for a quite a while without actually dividing it. When you divide, you can replant several of the same kind to give the same appearance of a large clump. Each cultivar seems to have it's own way of growing, and some just seem to grow nice and don't get too crowded for a long time, others seem to grow all over themselves and need thinning more often. So the answer to your question is "It depends on the individual iris" and your preference to how large an area you want covered with one kind of iris.