When I did it, I was told the "general rule" is that you should still be able to see through it on the bottom of the tank. Maybe someone else can shed some light on this? I know there was a site somewhere that had all that info with pictures of someone setting up a new planted tank. They added "mulm" from the filter of an established tank, on top of the peat to add nutrients. Then they used whatever substrate on top of that to "cap it off" so to speak.
Hope this helps! If I have given any bad advice, someone please correct me!