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Pages are only updated when certain fields are updated (title, description, keywords, modifiedDate).
The pages must be visited (i.e. the page and the script are loaded) within 30 minutes of updating or being added for it to be indexed.
Additional details:
Behind the scenes we check every 15 minutes to see if a instant index ‘ping’ has occurred. If the conditions above have been met the request then enters the crawlers queue, so the actual indexing usually occurs within 1 hour of the original request.