Are allergic reactions always immediate?

Depends: An allergic reaction usually occurs within 20 mins after exposure to an inhaled allergen but it may occur as late as 2 hrs with food allergy and 6-8 hrs with red meat allergy. For allergic skin reactions, it is usually from a different immune mechanism when the contact is on the skin. The reaction may not develop until 1-3 days after the exposure (poison ivy is one such example).

Not at all: Because there are many different immune mechanisms(the specifics aren't important in this setting) that cause "allergic" reactions, you can have immediate reactions on the order of seconds all the way to weeks out(like in the case of some drug rashes). It really depends on the type of allergic reaction you are referring too.