Actually, I'd like to add a caveat... The implications in the Main Scenario make it seem likely that the Broodmother's grief-stricken summoning of Lakshmi was the VERY FIRST instance of Lakshmi being summoned in her "hostile" form. When inquiring with the Vira about the summon, they note that Lakshmi is a benevolent goddess. They worry that the Qalyana may have summoned a twisted version of their goddess and warn us to be cautious when we go to treat with her - but if it was something the Qalyana had ever done before, it would be more than a concern, it would be an expectation.

So, I believe that Lakshmi as we encountered her in game was the very first hostile Lakshmi summoning, and those Qalyana tempered by her were the very first thralls of Lakshmi. Any tempered guards would likely have been tempered AFTER this encounter. So, we're looking at prep time of weeks, not years. The Qalyana must have been subtle and clever indeed to pull all this off without anyone noticing! Though, given the chaos involved in the struggle against the Garleans, perhaps it wasn't that uncommon for soldiers to go missing and then turn up again later - some of them tempered by the Qalyana.