Depends. Arts like Judo and BJJ are heavliy competency based. You learn the system, and then demostrate what you know in fully resistant sparring. Depending on how well you do determines if you get your rank or not. As such, it's hard to give a time scale on when you get your black belt in either art. In BJJ it will take most people at least 10+ years to get there black belt.

Aikido follows a similar grading system to karate, and Judo follows a similar idea once dan grade has been reached (i.e. you have to weight certain time periods between testing for the next dan rank).
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"Let your food be your medicine, and your medicine be your food" Hippocrates.