Back on topic.
This is O'Sensei's opinion for how long you should be training before Shodan(lowest grade, first grade):
"In your training do not be in a hurry, for it takes a minimum of ten years to master the basics and advance to the first rung. Never think of yourself as an all-knowing, perfected master; you must continue to train daily with your friends and students and progress together in Aikido"- O'Sensei
I would strongly suggest doing research about Ueshiba.
For instance, Mochizuki started training with Ueshiba around April 1931. Mochizuki was awarded the second highest rank/scroll from Ueshiba in June 1932.
Tomiki started training with Ueshiba around 1926-1927. He trained off and on until around 1934, when he moved to Tokyo. He trained until 1936 when he moved to Manchuria. Tomiki received 8th dan from Ueshiba in 1940. Start to 8th dan in 14 years. (And that's not even getting into the actual training time Tomiki spent with Ueshiba in those 14 years.)
There are other instances of Ueshiba just handing out rank when you dig into the research.