Den var bortglömd i årtionden. / Under årtionden var den bortglömd. (It was forgotten for decades.)
Christian Dior var den ledande modeskaparen i årtionden. / Under årtionden var Christian Dior den ledande modeskaparen. (Christian Dior was the leading fashion designer for decades.)
Notice that I use under årtionden to start sentences, not when for decades appeares in the middle of sentences. I don´t know why I do this, but it is how I use my native language.
It was hypothetical, but I guess it could be "Fashion took a hit in the seventies. This decade is best forgotten!" (where "this decade" is in the past) or "This decade I'm going to get a degree and move out of my parents' house." (where "this decade" is the current one).