Code Review: How long is a peer code review?
The code review process has been researched and studied since 1976, and it has be established and re-established that the maximum product code inspection rate is 200 lines of code per hour.

Quote:
“This is direct and conclusive evidence that reviews should be limited to around one hour, not to exceed two hours.”
– Jason Cohen

Application:
I’ve observed that the average developer creates around 100 lines of code per day in an agile environment. So I expect the average initial review in a peer review to last about 30 minutes. Now since there is usually a cycle of inspection, clarification, and rework, the end to end process of a peer review will vary based on the amount of back and forth clarification communication and the amount of rework that needs to be done. Teams should plan accordingly.

References:
http://smartbear.com/resources/whitepapers/best-kept-secrets-of-peer-code-review