The longer the paper or the more complex the criteria, the longer students will take to complete a thorough peer review. If you assign shorter papers, you can easily devote a part of a class to peer review or ask students to complete the peer review outside of class. But if you assign long, complex papers, consider breaking the peer review into several short chunks. For instance, students might complete one peer reading looking just for problems with focus, another for weaknesses in organization and development, and still another on graphics. Finally, students might have one or two additional peer-review sessions devoted exclusively to mechanics.
If you're concerned about taking so much class time for multiple peer reviews, consider the alternatives outlined under "Do I need to give students class time for peer-review sessions?"