Might not even take that long. Sr. Brigid is 75, and I don't believe there are many in her order who are much younger than she. Since the 1980's, and the nuns started getting into the 'Social Justice' arena, and moving away from Church teachings, they haven't had much in the way of novices. Why would a young woman with a desire for a religious vocation be drawn to that?
The Communities of Religious Women that have been growing since the 80s are the ones that have stayed close to the Church, and several new Orders have begun in just the last 10 years, or so. Most of the Sisters wear habits, some less elaborate than others, but it's clear that they love God, and want to serve Him. Unlike their elder sisters, they don't seem to consider teaching and nursing demeaning.