The safe duration of allergy medications depends on the type of formulation. For instance, for the first generation antihistamines, it is up to 7 days while the second generation antihistamines can be taken for up to one year without interruptions if they are well-tolerated. But you should keep in mind that in the long-term use of the formulations the toxic effect on the body is possible and even allergy to antihistamines can develop.