If you set your review cycle up with a manual sequence, then you do not need to do anything to extend the review cycle. The timeline is fully depending on manually moving the review through the cycle.
When you are ready to end the review cycle:
Step 1: Navigate to the "Admin" Panel found on the top navigation bar
Step 2: Click on "Reviews" found in the left Administration Panel
Step 3: Find the review you would like to extend and click "Manage cycle" to the right of their name
Step 4: Click on "End Review Cycle"
If you set your review up on with a timeline sequence, you have the ability to manage all the end dates of each part of cycle from within the review. To do this: navigate to the "Admin" panel, click on "Reviews" found on the left, and "Manage cycle" for the review in question. You will see the cycle that you have scheduled, along with the dates and the ability to edit those dates.
You also have the ability to take manual control of the cycle, which will move it completely off the timeline and the review will end when you are ready.