The database is indexed after about 5 minutes of inactivity following any recent modifications, in case there are several users making changes in a database at a time (which is common).
If there is no 15 minute break in activity for over 4 hours, the case will be indexed so that the database can have a partial index available.
If the search index is out of date a cautionary icon will appear adjacent to the search bar. Clicking the icon will provide you with more information. If the cautionary icon is not visible the search index is up to date.