Someone ask this question recently via Facebook, and my response was as follow:
Some will say that the completion stage starts after receiving the blessing. However, it may not always be the case. One should be able to recognize on their own when their relationship with Heavenly Parent has reached a new level.
My personal experience has been as follows: After the departure of my husband to the spiritual world over 12 years ago, I found myself alone for the very first time in my life. My daughter (and only child) was all the way in California, and my husband was gone. For the first time in my life I was able to think of "me". So, I sat in my living room, in the evening, near my altar and just did nothing. I will look at the altar, I would think of my husband, my daughter but also I will think of me. Pretty soon, I started to read one by one all the books printed about True Father and True Mother's lives and talks. I read all of them, some of them, several times.
12 years later, I realized that my ability to relate to God and understand True Parents most definitively grew tremendously. I was not just a Unificationist because I believe in the Divine Principle. I was a lot more. I could relate to God and think of True Parents with a lot more "true love" not mandatory love.
I believed that I am now in the completion stage of my life. It's about time at 73 years old and 52 years as a Unificationist
PS: According to the teaching of Rev. Sun Myung Moon, we have 3 stages in our lives:
Formation, Growth, Completion. Not all of us reach these levels at the same age, it is a matter of discovering who we are in relationship to the creator our Heavenly Parent (God, the almighty), our relationship with each other, and our relationship with the all creation.