It's standard in most arts that access to rank/belt exams is restricted in some way. In many cases, there are minimum waiting periods between exams, and in some dojos, an exam can only be attempted with an instructor's recommendation.
Is there a style or system in which anyone (or, at least, any paying student) may test for the next (or perhaps even any) rank whenever they reasonably wish to do so?
In the academic world, this kind of concept is becoming more and more a thing, with some universities offering course credit (or even entire degrees) for passing challenge exams that can be taken "anytime", but I admit I've never heard of any dojo that would grant a belt to someone who just showed up, paid a fee, and passed a test (or even a battery of tests).
If the answer is no, is such an arrangement inherently inimical to the spirit of martial arts (i.e. even if you can pass all the tests, you aren't really a Master until you have spent X years sweating and socializing in the dojo), or is this more of a practical matter in that very few students have what it takes to go any faster than normally allowed, so instructors tend not to encourage it?