Typically, up to 12 weeks (yes, 12) after removal of the allergen. You would probably see some improvement in symptoms within that time (more so if they weren't terribly severe in the first place, or you weren't very observant). But if its a problem that has persisted for some time, and has produced quite severe symptoms, then it may be several weeks before you really see any change, and the full 12 weeks for complete dissipation.
That is why the standard length of time for food allergy trials is 12 weeks duration.
It is also worth bearing in mind that allergy testing isn't all that reliable for food allergies (sorry). Irrespective, the way to prove or disprove an allergy theory is to cease feeding the suspect item, and to swap to something with entirely new ingredients (as in, stuff the dog has never eaten before). That new, unique, food needs to be the sole thing the dog eats for an entire 12 weeks. If the symptoms go in that time, you've proven an allergy existed. If they don't go in that time, and providing the new food really was new - then you've proven the reverse (that a food allergy does not exist... OR that he is also suffering from another problem).
