Yesterday in that Google Hangout on Google+ with John Mueller, John said at the 22:28 mark that Google updates schema, structured markup and rich snippets almost immediately. Basically as soon as Google recrawls and reindexes the markup, Google can immediately show the rich snippets in the search results. Of course, Google doesn't always show rich snippets for all sites because of their filters but if they do already, it should be quick to update.
Baruch Labunski asked:
Regarding schemas, how often are the updates? So suppose, you know, you change certain things for this ecommerce website. Yeah, I mean they have you know a hundred reviews and they had like 60 or whatever in the rating. Basically let say you make some changes to your schema, how long would it take for that to take effect?
Google's John Mueller responded:
Usually pretty much when we crawl. So we have to recrawl and reindex those pages. But it's it's kind of like content on your pages where if you change the content once you've we've been able to reindex that updated content we should be able to take that into account immediately. There's no additional delay from there.
Here is the video embed:
Forum discussion at Google+.