There really isn't a set standard at most places, I think. It's when your instructor feels it's right, is when a promotion occurs. When I got my fourth stripe at white belt I got "the talk" from Rodrigo that went something like "Ok, now it's time to compete, then you get your blue. Poa!" (AKA, you've been a blue belt for 6 months and I want another trophy). I went, competed, did horribly... Like, megabad, should never put on a gi again horrible, right in front of Rodrigo. I was pretty devastated and resigned to the fact I probably just delayed my promotion, but within a few months, I tested for my blue.
Conversely, I've seen guys dominate the shit out of people and don't get any recognition at all other than a "Poa, buddy, great job. You should have cross gripped more from guard, though poa."
And at the end of the day, who cares. It's a platitude at this point, but the mat doesn't lie. Deep down, you know where you're at and don't need a belt to tell you.