To answer your question thoroughly we really need to discuss what the purpose of a grit progression is and what is happening to steel at the edge of the razor when you hone.
Basically you are wearing away steel with the abrasive particulates in a hone when you sharpen a razor. These abrasive particulates leave gouges in the steel, and not all of them are uniform in depth or width etc, particularly at lower grits. People often refer to striations in the metal, or a scratch pattern.
The generally accepted purpose of a honing progression is to get the two sides of the bevel as close as they can get to each other, and get those two bevels as smooth as they can get as well. In other words, try to smooth out those gouges made by the lower grit stones. As a side-effect of that smoothing process you are also generally speaking bringing the two sides of the bevel closer to each other as well, so that's a good thing.
So then, to answer your question. You should progress to the next stone when the current stone has reached its optimal cutting and smoothing potential. So unfortunately you don't know when to progress to the next stone. None of us do. We guess, basically. An informed guess, for sure, but a guess it is.
People with more experience are better guessers than those with less experience and therefore produce more consistent results. This is why beginner honers can get a great edge on one razor, but then cannot repeat it again - they fluked a good guess.
To aid in the guesswork we can use subjective empirical tests. Onimaru pointed to the Wiki link for a description of those, and it is well worth reading them. But just remember they are subjective - you need to work out how they work for you, which means lots of experimentation with lots of razors over lots of time.
The best guessers out there are called Honemeisters. It's barely a guess with them to be honest, but nevertheless, when talking about an optimal process, grit progression always has an element of guesswork in it. It's why some people talk about intuition, and why it is so hard to explain in words how to hone.
James.