I can't speak for you, nor anyone else with a healty eye, but I can say this; after my last 3 fundus exams, I walked out of there with a permanet vision reduction (progressively worse every time). Now I refuse any more tests. After a fundus photo, the inverted image of the flash, stayed in the eye for 2-3 hours.
Probably up to 30 minutes. It is just a guess.
Dr. O.