Cetirizine hydrochloride oral syrup. How long can it taken?

Indefinitely. There is no limit on how long any antihistamine, including cetirizine, can be taken. Antihistamines work by blocking histamine receptor sites and are most effective if taking regularly. If it is not working, another medication may be tried or it may indicate more moderate to severe allergies. See a board-certified allergist if this is the case.