Review Rating Script when it will appear in a rich snippet ?

When Google next crawls the page. However, just because you've added it doesn't mean Google will allow it. In some cases, they simply won't show your stars.

To speed up the process, go to your search console and request Google to crawl the page in question.

1. Add the required properties. ...
2. Follow the guidelines.
3. Validate your code using the Rich Results Test.
4. Deploy a few pages that include your structured data and use the URL Inspection tool to test how Google sees the page.

Generally it takes anywhere between 2-12 weeks depending on many factors. It should also be noted that Google can decide 'NOT' to display your rich data in search results. Having Rich Data on your website isn't always a golden ticket to richer search results, it varies from site to site, and niche to niche.

A few things to note to help Google display "Starred Review" listings in the serps.

1. Do not put review snippet code on home page.  This will get your site penalized
2. Do not add this code to all of your pages at once.  Slow drip adding all pages that qualify.
3. Put some of your reviews physically on the pages that the snippet is place one.  If you don't you may not show up in the serps with review stars for pages with no reviews on them.
4. Only list your Google reviews.  If you don't have any, earn some.

If you haven't violated any of Google's TOS then 4 to 6 weeks has been my experience before you see any results.