Well from my own observations I would say that orange belt (the third belt) is often the dividing line where students become serious. Belt systems of course vary from dojo to dojo in terms of colors and the order that they go in but its common, at least from what I've seen, for orange to be the third belt where the order goes white, yellow, orange. I would say many, if not most students, don't make it to orange.
The martial arts has a very high turnover rate. Lots of people try it out for a little while, just to see what it's like, and then once their curiosity is satisfied they'll move on. As such many students will drop out at white belt, or they might decide to get their first belt after white belt which is often a yellow belt and then drop out after that. This is just what I've seen that many students will just try it out for a little while and maybe go up one belt and then decide it's not for them. As such, you could say that orange belt (or whatever the third belt in your system is) is the dividing line.