The first thing you should do is to make sure that you’ve configured the ratings’ rich-snippets correctly. Go back to the this topic and follow the instructions. Once the configuration is right, usually it takes about two weeks until Google will add the Rich-Snippets ratings. It’s really varies according your store’s content quality, crawling frequency, pagerank and many other parameters. Even though we have many clients that the ratings appeared on their SERP, we can’t assure that Google will eventually add the ratings’ rich-snippets. Unfortunately we don’t have an access to their algorithms.