I have never understood the rationale behind time requirements for tests, at all. I think people should test when they are ready - by the standards of their organisation, and by the judgement of whoever puts them forward for their test.
But that's a tautology, isn't it? You should test when you're ready -- that makes perfect sense. But how do you know when you're ready? Well, by the testing criteria! So, the way that you know you're ready for the test is if you've...already...taken the test?
The rationale really isn't hard to understand. Training time is an arbitrary standard that does not perfectly reflect the skill and readiness of the person to whom it is being applied -- I don't think USAF has ever said otherwise. But those who focus on the imperfection of that standard can never produce an alternative that's any better. So, it's what we got -- not perfect, but also a long way from useless.