Six years. Rosa had her first vision about Pryde while she was pregnant with Tiara. (For that extra hormonal bonus.)
According to that chapter (chapter 5 in the manga, 18.1 in the novel), Rosa has several visions a month, and she's predisposed to seeing Unavoidable Futures (ie, futures that cannot be changed--and she's tried before. Stopping an assassination only ended in the victim dying in an accident the next day). So Rosa has been seeing Pryde outright torturing people in her visions a minimum of three times a year for six years.
I guess that is a much better explanation. In that case, I wish the manga would touch more on that instead of leaving the readers with the impression that the mom is stupid and terrible.
I wish that the manga would cover Rosa more, period, because the abuse she's been getting from the fandom just isn't fair. (And this is coming from someone who dislikes both Pryde's parents and her uncle.)
But given how rarely Rosa appears in the novel (only appearing in 15 arcs out of the current 31, often only for one scene), I doubt they plan to give her background much focus. Got to move that plot forward, and the Tragedy of Rosa is left for us novel readers.
Wow I love the FL so much!
I really hate the logic of her mother though. Why tf would you neglect your daughter for 8 years just bc u had a premonition that she'd be a villainous? Shouldn't you give her love and bring her down the proper path?