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 sections. 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 assignment-specific elements such as use of data or graphics. Finally, students might have an additional peer-review session devoted exclusively to mechanics.
Also, think about the big picture of your syllabus. Students need enough time to complete the peer review and revise before the paper is due, so work backwards from your intended due date to schedule the peer workshops.
Visit the following links to view how peer-review instructions might vary according to the time you have allowed for the peer-review process:
Sample Workshop Checklist--for short, in-class workshops
Arguing Essay Peer-Review Sequence—a series of worksheets designed for multiple rounds of review on the same essay