Objective Guidelines have been published by the Food and Drug Administration (FDA) and the European Agency for the Evaluation of Medicinal Products (EMEA) for the conduct of seasonal allergic rhinitis (SAR) and perennial allergic rhinitis (PAR) studies. These guidelines have differences regarding the duration of such trials: the FDA suggests 2 weeks for SAR and 4 weeks for PAR but the EMEA suggests 2 to 4 weeks for SAR and 6 to 12 weeks for PAR trials. In the interest of global harmonization, it would be desirable to have a uniform duration of such trials so that investigators, internationally, would be able to readily compare results for various types of treatments based on a single standard. Therefore, we performed an evidence-based review to answer the clinical question, What is the optimal duration for SAR and PAR clinical trials? Methods We performed a MEDLINE search of the published literature from 1995 to the present. We used appropriate search terms, such as allergic rhinitis, seasonal allergic rhinitis, perennial allergic rhinitis, SAR, and PAR, to identify pertinent articles. These articles were reviewed and graded according to the evidence quality. Results After an initial screening of more than 300 articles, 138 articles were analyzed thoroughly. No study specifically addressed the question of the optimal duration of SAR or PAR clinical trials. Conclusions We conclude that the current FDA (draft) guidelines calling for a study length of 2 weeks for the assessment of drug efficacy for SAR and 4 weeks for the study of drug efficacy in PAR are appropriate and that longer study periods are not likely to add meaningfully to the assessment of drug efficacy.