hi jerry. do you mean fully open? or start to open from a tight bud?
mine start to pop open in the cooler hours of the early morning or hours close to midnight. but, they don't fully relax until there's some warmth of the day. sometimes, if the iris are a complex flower with many ruffles or lace, they have taken a-day-and-a-half to fully relax. anyway, that's been my two year experience in southern california.
whereas, when i lived closer to san francisco it seemed to be overnight. however, iris forms were simpler then, and not as many frills and ruffles.?? just seems to me imho.😀