Good question, Lisa.
I say about once a year, depending on where they are.
Mine are mostly not stored away and because of my cats I like to wash. But air drying in the dryer is a good idea too.
I have my heirloom quilts in a quilt curio chest. They are special quilts I bought: I didn't make them. I air them out occasionally on my clothesline.
My holiday quilts get washed and stored away after the new year. You just reminded me to get them out!