There is no maximum , all gradings are upon invitation including Shodan grade and under no circumstances is a student to ask when or for a grading.
This is for the Chief Instructor (Kyoshi) to decide and those that will be grading will be informed .
Novice white belt 10th Kyu
Red belt 9th Kyu
Orange belt 8th Kyu
Yellow belt 7th Kyu
Green belt 6th Kyu
Purple belt 5th Kyu
Purple belt 4th Kyu + 1 stripe
Brown belt 3rd Kyu
Brown belt 2nd Kyu + 1 stripe
Brown belt 1st Kyu + 2 stripes
6 months to 12 months depending for
Black belt 1st Dan (Shodan)
Although the general rule is a minimum of 3 years regular training before a student can grade to Shodan this is indeed very rare but can be done depending on the ability , knowledge, and standard of the student/s concerned!
Normally most students average 4 to 5 years regular training 2 times per week to reach the level of Shodan but can be longer depending on the students progress etc and attendance in their training.
Junior Shodan gradings will not be taken before 13 years of age , and upon reaching 16 years of age will retake an assessment before advancement to adult Shodan grade .