Excellent to divide immediately after bloom. If you have written the color on a leaf with a sharpie It will not have time to fade and you can toss extra duplicates. They also have lots of time to re-establish. How often depends upon the cultivar and growing conditions. Vigorous ones may need dividing every 3 years. Others maybe 5 years. I like to have at least 2 feet between the different colors. Say I have a vigorous one called New Moon. I'll keep 3 nice rhizomes with leaves and plant them in a triangle with the leaf end facing outwards and the non-leaf ends touching each other (they grow outwards from the leaf end). 2-3 feet away I'll repeat with another color.

Have you ever divided before? This pic shows how they spread/travel. The newer rhizomes, with leaves, coming off the old mother rhiz. are the ones to save. This iris doesn't have a very neat growth pattern. .

The rhizomes like to sunbathe, so don't bury they altogether. I bury mine about halfway and then make sure to remove any leaf litter by hand (no rakes) in early April. I have an old iris bed and only dig up sections where there are big holes in the middle and try to stick with just that spot. They really do go years and years undisturbed and keep blooming.