Two quick examples of post war students. Many students, both prewar and postwar, had training in other various martial arts before studying aikido. As Carl Thompson noted, some did not and still were promoted quickly.
6th dan 1970
4 years start to 4th dan.
12 years start to 6th dan.
8 years start to 4th dan.
These guys were uchideshi, they trained 7-eleven. Chiba trained 6 months in Iwama. The 6 months training is equivalent to our 3 years' training.