How long did the silver vein near Earsome remain productive?
Here's what the lore says, drawn directly from the indexed sources:
-1045 DR * A vein of silver is discovered near Earsome, causing a brief population boom before it is played out within the next 86 years.
Source: -1045 DR
-104 DR * After suffering innumerable orc raids, the dwarves of Delzoun abandon Besendar's Blockhouse. * In in Serôs, the Emerald Eye of Kyron is studied for a time before it is hidden and sealed inside the black coral walls of Vynagyr.
Source: -104 DR
-1069 DR * The Orcgate Wars end as the orc horde and their gods are finally defeated in the Priador and scattered throughout the region. * Following the war, the Nar, Rashemi, Raumviran, and Sossrim mercenary tribes make their way home. * The Nars establish a series of small kingdoms over the course of a century. The greatest is Ashanath by Lake Ashane.
Source: -1069 DR
History "Rock Eaters" They are Not! was written and printed sometime between the citadel's construction circa 900 DR and its fall in the Battle of Seldarine's Hand. By the, the citadel was ruined and inhabited by ghostly apparitions of its former inhabitants. One copy could be found in the citadel's library curated by Custhantos, along with many other rare tomes.
Source: "Rock Eaters" They are Not