The Scourge lasted several centuries. As the magic level died down most of the horrors were forced back out of our world and back into theirs. Some Fourth World civilizations survived the Scourge thanks to Theran Empire's development of kaers - underground, warded cities. Even though kaers were purposefully designed to survive the Scourge, many failed (An estimate of a 50% failure rate may be a good rule of thumb). Some communities were protected by dragons. It is unknown if there are other ways to survive the times of Scourge.
The Scourge also affected the land - as some Horrors simply feed on living matter (or emotions of non-intelligent creatures), much of the ecosphere was damaged; Earth in early post-Scourge years was not too far off from post-Apocalyptic realm, although it has seen some relatively quick recovery, aided by some dedicated magic (druids, etc.).
Presumably, Scourge-type events happen every time magic levels peak. It is unknown how Earth civilizations survived other Scourges, it can be suspected that dragons - who have lore dating at least to the Second Age - know of some methods. All Scourges seem to have been devastating.