In most cases, anaphylaxis will occur within 20 minutes to two hours of exposure to an allergen. Some reactions can be more delayed and may only manifest with symptoms one or several days later (making it difficult to identify the allergen culprit).
Some people may experience biphasic anaphylaxis in which primary anaphylaxis symptoms will come and go, only to be followed by a severe secondary attack within 72 hours of the initial event.