Rokudan in Aikikai
In Aikikai, Hombu rules lay out a minimum timeline: 1 year from shodan to nidan, 2 years to sandan, 2 years to yondan, 5 years to godan, 6 years to rokudan. A total of 16 years from shodan to rokudan. Observe that these are _minimum_ requirements.
A commonly applied minimum, in several countries as well as several budo arts, is:
5 years to shodan
2 years to nidan
3 years to sandan
4 years to yondan
5 years to godan
6 years to rokudan
A total of 25 years from the date of starting aikido.
Reaching rokudan significantly faster than this is rare.
In Aikikai the grades from 5 dan and up are recommended only, so there is no testing. The requirements are not spelled out anywhere that I know of. Of course, you have to be practicing aikido continuously, showing improvement - and you have to be recommended by a shihan or a grading committee.
Rokudan is the shihan level, implying a capacity to teach students this do properly - lead them on the way, so to speak. In Aikikai you are not shihan automatically, when receiving rokudan. It has to be applied for, and among the additional demands is that you've had 6 dan for at least 6 years.
What kinds of qualities and capacities a rokudan is supposed to have, apart from the above, I guess I should know, being one myself, but I have no clue. Well, one: It is mainly up to the person having the grade to live up to it, and to figure out how.