Minor quibble - Aikikai grading criteria are minimum 1 year to nidan, 2 to sandan and 3 to yondan, but with practice days requirements as well:
Your stated minimum are the more practical levels.
I know. That's the Hombu minimum, extracted from a dojo with bundles of high grade teachers and practice five times a day. In such a dojo, this quick advancement is possible (although still not recommendable, in my personal view).
The periods between dan grades that I listed are, as you say, common practice - not only in aikido outside Japan, but in other budo as well.
Also, I kind of like the mathematical symmetry of it - sort of like a question in an IQ test