The timeline for submission withdrawals is determined by the Program Chairs of each venue. Therefore, there is not a single overarching OpenReview withdrawal policy. If you have questions about when in a venue workflow you will be able to withdraw your submission, you should reach out to the Program Chairs of that venue directly.
For Program Chairs
Authors are able to withdraw their paper at any time after the submission deadline or after Post Submission Stage has been run. You can optionally restrict the withdrawal window from your venue request form. You can also use the venue request form to configure the visibility of withdrawn and desk-rejected papers, as well as the identities of their authors.