How often do you grade?

We have six kyū ranks before black belt (that are called dan ranks). Normally, you can do a grading test after about 3-4 months for the first grading, the higher up you go, the longer it is between each grading. But it can go both faster and take longer time, it depends on how much you practice. There are minimum requirements for a certain number of training sessions, and a minimum time in months, since the last grading before one are qualified to do the test for the next grade. But the primary thing is, of course, that you know and can do what is required, both technically and theoretically, in order to pass the grading test, not only that you have been practising for a certain time.