Where I train, for an adult it's normal for it to take between 4 and 4.5 years. The grade order goes like this for us : White --> Blue --> Red --> Orange --> Yellow --> Green --> Purple --> Purple (white stripe) --> Brown --> Brown (white stripe) --> Brown (red stripe) --> Black. However from the yellow belt onwards, under 14s are required to grade for a 'tag'. So they would have a yellow belt with a green tag on the end. Then the next time, they take their green belt grading. Then green belt with a purple tag etc. This adds quite a bit of time to the goal of Shodan. Especially with the purple/brown belts! As they sit a tag in between each one! I'm sure other schools will vary greatly, but that's just the system I'm used to.