The exact duration of the Scourge was recorded differently by isolated cities and cultures, though each marks its beginning by the same event; the sealing of Thera (1008 TH on the Throalic calendar, TE 565 according to the Theran calendar).
As the magic level died down most of the Horrors were forced back into their world. Some Fourth World civilizations survived the Scourge thanks to Theran Empire's development of kaers - mostly underground cities warded by the living earth surrounding them. Even though kaers were purposefully designed to survive the Scourge, many were penetrated by the Horrors. Some communities were protected by dragons. It is unknown if there are other ways to survive the times of Scourge, though there are unconfirmed reports of isolated communities that somehow survived (such as Ogres and Cave Trolls).
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 (Questors of certain Passions etc).
Fragmentary records exist indicating that another Scourge occurred in a previous Age. Presumably, Scourge-type events happen every time magic levels rise. It is unknown how Earth civilizations survived other Scourges, though it is suspected that dragons - who have lore dating at least to the Second Age - know of some methods. All Scourges seem to have been devastating.