Just find the exact date that Rutherford declared that there was a great crowd and you will find that they started to form instantly in that same moment.
From http://www.letusreason.org/JW52.htm
At the “1935 convention in Washington, D.C.” the leaders of the Watchtower publicly proclaimed to the Jonadab class they were to inherit an earthly paradise as their eternal reward (Jehovah's Witnesses In The Divine Purpose, p. 140).
The doctrine introduced by Judge Joseph Rutherford, second President of the Watchtower said from 1935 onward, “At the end of May of that year a five-day convention of Jehovah’s Christian Witnesses was in session... the president of the Watchtower Bible and Tract Society discussed the subject of 'the great multitude, 'spoken of in Revelation 7.9-17, Authorized Version. He made it clear that the 'multitude' was no spiritual or spirit begotten class, would not attain the angelic nature in heaven so as to assist the 144,000 joint heirs with Christ. It was a distinctly an earthly class with hope of endless perfect human life in the earthly paradise under Christ's kingdom” (Holy Spirit, 1976, p. 156)
Everyone who became a Jehovah’s Witnesses before 1934 would go to Heaven (until Rutherford changed it). God stopped calling people to only a heavenly calling from that year. So he invented two classes, a heavenly class and earthly class. The “great crowd” class introduced by Judge Rutherford out of necessity. An earthly Great Crowd was created, a second- class of people that were subject to and dependent upon the heavenly class who alone would be part of the new covenant..........
Rutherford changed the sealing of the 144,000 from the original date of 1881 to 1931. “God having a fixed time for every purpose (Ecclesiastes 3:1), his time to give creatures on earth the opportunity to get in line for a heavenly reward has been from A .D. 29 until, chiefly, 1931, called the “day of salvation. It began with Jesus at Jordan and is now rapidly nearing its end”“ (Let God be True” 1952 2ed. p.298).