The MacRumors report, from a couple of weeks ago, says that changes are being implemented weekly. The Reddit thread says they’re happening nightly — at 3 a.m. Eastern:
Over the past month, Maps were being updated once a week (every Friday) for me, but now, that has improved further. Over the past few days, I’ve noticed an update occur every single day at 3 a.m. Eastern. At exactly 3am, Apple pushes out new data and corrects many of the problems that users had previously reported. For example, I noticed that if I open the app at 2:59 but then again at 3:01, the map would reload itself, and a bunch of POI corrections and updates would suddenly be present.
Apple would not comment officially about the accuracy of these reports, but did point us back to the feature (introduced in iOS 7) that allows users to submit corrections or changes in Apple Maps.
Although Apple wouldn’t verify that the updates were in fact happening (or the schedule) it appears from these third-party anecdotal reports that the company is stepping up the frequency of changes and corrections.
While the initial rollout of Apple Maps was botched, the company has subsequently taken pains to improve the quality of maps and their POI/location data, buying a number of companies (e.g., Locationary) to aid in that effort. In addition, iOS 8 promises a number of data upgrades and feature enhancements, such as new flyover “City Tours.”