Mushinkan Kendo & Iaido Promotion Policies
The time between ranks required by the SWKIF is 6 months until you reach shodan (1st Dan).  After shodan the time between ranks is the same as your current rank in years, e.g., a person holding 1st dan is elegible to test for 2nd dan 1 year after passing 1st dan, a 2nd dan holder is elegible for 3rd  dan 2 years after passing his/her 2nd dan examination, and so forth. 

Mushinkan internal policy for promotion eligibility adds the following:
To apply for promotion you must have the minimum documented practice hours, be of the specified qualifying age or older, and have written permission from the head instructor.
Min Required Age
Documented
Class Hours
Rank
Challenging
6th kyu
5th kyu
4th kyu
3rd kyu
2nd kyu
1st kyu
Shodan
Nidan
Sandan
50 hours
75 hours
100 hours
150 hours
250 hours
400 hours
550 hours
750 hours
1150 hours
12 years of age
13
14
15
16
18
Hours can also be earned through participation in seminars and tournaments: Number of seminar hours for seminars and 5-hours per tournaments. 
You also get credit for attending class even if injured and cannot physically participate. 
Make sure your practice attendance is recorded.