15-30min. Antihistamines in general can take up to 30min or so to work. It should not take you more than a couple of days to know if that medicine will work for you or not.
Acts within 2 hours. Desloratadine (clarinex) has been shown to significantly reduce nasal congestion from allergen exposure within 2 hours.
Allergies. It may work immediately, it could take day or weeks. It depends on the enviroment/winds (some weeks are worse than other), the severity or mildness of your allergies. There is stepwise approach for allergy eval & treatment (many time requires combination of med, allergy testing etc. Depends on patient). Usually I do follow ups 3-4 weeks at time to allow med to work and weather to change.