how fleeting is evanescence?

How short or fleeting is the time span of Evanescence ? Can anyone give a definition in exact measurement?

I encountered this question when visiting an old abandoned/transformed mining town. How short is short? We just cannot tell. For the miners who used to live there during boom time, some might have thought the precious ores would last a long time in their own life span. Others might have joined much later and perhaps sensed time ticking away and soon they would have to move on. They were not alone. History does not change much. Any man would know soon that we are not getting younger. Man’s glory fades with time as the source we consider precious (in this case, valuable ores) depletes.

I quote excerpts from Wikipedia: “Virginia City was the prototype for future frontier mining boom towns, with its industrialization and urbanization. It owed its success to the 1859 discovery of the Comstock Lode. After a year in existence, the boomtown had 42 saloons, 42 stores, 6 restaurants, 3 hotels, and 868 dwellings to house a town residency of 2,345. At its height in 1863, the town had 15,000 residents. The mines’ output declined after 1878, and the city itself declined as a result. As of the 2010 Census the population of Virginia City was about 855. Today, Virginia City is but a shadow of its former glory…”

For those interested in knowing more about the historical and mining background of the above photos, here are some links to wikipedia:
https://en.wikipedia.org/wiki/Virginia_City_Historic_District_(Virginia_City,_Nevada)
https://en.wikipedia.org/wiki/Comstock_Lode
https://en.wikipedia.org/wiki/Virginia_City,_Nevada