Until the Pigeon flies are off. The loft must be kept very clean after treatment, it is also a good idea to spray the loft at the same time.
Pigeon flies can be gotten rid of, but again as George said, you must treat the bird, as well as keep the loft Scraped, and vacuumed, at least for two weeks after you treat the birds, again remember, the Pigeon Fly can leave the Bird anytime it wants, and wait for the treatment you have done on the bird to wear off, if need be. I would rather have any other External parasite on my birds, rather then them... Dave