In testing this functionality within Intune over a period of 6 months, I've found that Discovered apps is only being updated between 0 and 2 times daily. Furthermore the content displayed within Monitor | Discovered apps seems to differ from what the enrolled devices show, as can be seen if you click on an outdated item and are occasionally met with no results, with this same issue (sometimes) extending to data exports.
It's frustrating to see that many of the issues which were brought up nearly 4 years ago are still present today. Obviously we appreciate that the update frequency has been reduced from 7 days, but it is often not done within 24 hours. It would honestly make more sense if this were done in fairly real-time when devices check-in and when a sync occurs, but I suspect there's a very good reason why that is not the case.