"Labhraíonn siad ar feadh nócha nóiméad."
Translation:They speak for ninety minutes.
Despite what Duolingo might lead you to believe, do does not translate as "for" in every case. Especially with time. If the action is completed, or a designated time, use ar gead, followed by the genitive (ar feadh míosa). If it's ongoing, use le (Tá mé anseo le trí mhí anois - "I've been here for three months.")
When I get back to my binder I'll updated with more detail.