How long did the storm take to have such a massive impact on the general population?

Lore wise we know the storm took out 98 percent of the world's population, but clearly it wasn't an instant thing. I mean on walls in suburbs we see graffiti warning people to go home and graffiti of husks scattered around, not to mention the fact that clearly homebase and vindertech needed some time to just develop the tech for stormshields. And ofcourse cant forget the seemingly ancient cave paintings showing husks and mist monsters in caves. Is there any specific answer Asto how long it took for the storm to actually wipe out that much of the population?