I don’t think the Montfortian consecration has an expiration date. I believe St. Louis recommends renewing it annually to remind ourselves to remain faithful to it, and to profess our devotion to our Lady, just as we do with our renewal of baptismal promises at Easter Vigil.
I suppose if someone fell into mortal sin, or particularly apostasy, it would be especially appropriate to renew the consecration then. The whole point of True Devotion to Mary is that just saying you’re devoted to our Lady is not enough; our lives have to change.