What clinical evidence is there that an allergic reaction to food can
occur more than four hours after ingestion of the allergen? I have
numerous articles that suggest immediate reaction is typical and
reaction times up to
four hours can occur. I'm looking for solid research on whether it can
take as long as 12 hours. It would also be great if I could find out
how common or uncommon it is for a food allergy reaction to occur more
than 2 hours after ingestion.

An interesting question.  One problem with a good answer is that of
defining a food allergy; often people will say that they have an
"allergy" when in fact they have intolerance to the food, or they
merely incorrectly attribute some sort of physical symptoms to a
particular food item.  Medically speaking, an allergy is an
immune-mediated event, that is to say, the immune system reacts to a
foreign substance.

There are four types of hypersensitivity reactions.  Without getting
into too much pathphysiologic detail here, most food allergies are
type I, or immediate hypersensitivity reactions, but some are also
type IV or delayed hypersensitivity reactions.  Type I reactions are
like the classic peanut allergy: you eat a peanut, your lips and
tongue swell up and you develop a rash within minutes, then your
airway closes up and you die.  (Ok, not all of them are that severe). 
Type IV reactions are like your classic poison ivy; you brush by some
poison ivy, and several hours later, you start to itch.  Some people
(especially those very sensitive to poison ivy, incidently) can have
similar reactions to mangos (the oil on the peel, I believe).

If you're interested in incidence and prevalence, searching for type I
and type IV hypersensitivity reactions would be a good jumping-off
point.