That's the question you're all asking... It's not really possible to give an answer as the timing of the flight season depends heavily on weather during the pupal stage: they can shoot through this in 16 days (or perhaps less in very hot weather) or get stuck in it for 26 days, as happened last year. Also, the process of pupation can be prolonged by cool wet weather.
At present, wild larvae in and around Savernake Forest in Wilts are in exactly the same state they were in this time last year, and only fractionally behind this time in 2010: some are fully grown, though their feet haven't turned pale yet (which happens immediately prior to pupation), some are in the mid-final instar stage, but one late one has only just entered the final instar. Savernake is a 'late' iris site, the insect will be more advanced at 'early' sites in Sussex and Surrey. Here's two final instar larvae (one late-final, one mid-final) that have fed close together all spring (spot the caterpillars):-
My guess is that iris will start in Savernake around July 7th but will be out in early sites such as Bookham Common at the very end of June - but so much depends on weather during the second half of June. My advice is to book your holiday week for the second week of July (that's what I'm doing).
Watch this space...