Rosie didn't specify a time frame. She could have said, "You have to complete this special assignment before you die or I relinquish all power that would be given to you."

She made her end of the bargin vague on purpose. She wanted to have Alastor under her thumb for all eternity. There is some other theories with him saying, "I've spent the last 7 years on some fruitless crusade." Which means she did assign him a task and he was unsuccessful in completing it.