1 class for skill retention (2 hr), 2 class for steady progression (4 hr), 3 to improve (6 hr), 4 classes to advance quickly (8 hr), 5 classes or more (10+ hr) and your pretty much uchi-deshi.
This week I trained for 6 hr with 2 more weekend, so I'm advancing the fastest. I'm also the most injuried.
People who ask for minimum class to get a belt will never get better. They think it's like getting a license or cramming for a test. Martial arts is hard work and stready progression. People who thinks they can cut corners in training won't last long anyhow, since they never self train, they don't improve fast either.