While we have always heard "can take up to 24 hours" over 25 years that has never been the case.
Perhaps you have been lucky to only list in categories that are not subject to extra scrutiny, or where traffic allows almost instant indexing.
While there is no doubt that some listings are indexed almost immediately, not all are, and there are many others that do not appear until nearly the full twenty-four hours has elapsed since the last revision; and a few outliers may take even longer, particularly for new sellers or sellers listing in certain categories for sporting goods like hunting rifles, or for certain kinds of militaria.
