Usually every 3rd. year. But it all depends on how big of a clump you want, how much space you have, the variety itself. Some like Fogbound are almost weedy in how well they increase, others are much slower.

I agree, every 3 years to dig and divide most varieties of bearded iris. Some of the broken colors do not throw increases at the same rate that the other cultivars do.

Thank you! Is it recommended to cut off the spent blooms? Will it reseed itself, or it multiplies by rizomes only? Please excuse my ignorance in this regard, I do not know much about irises, though admire and appreciate their beauty.
~Natalie

Cut the spent stalks, grooming the stalks makes the remaining blooms look better.

Will do! Thanks.