The forum has a standard post length of at least 4 lines. (On a 1280 pixels wide screen, is what I use for deciding this. To check for sure I recommend simply opening paint, creating a 1280 pixel wide blank image, and resizing your browser window to be as big as the image.)
Individual RPs generally don't raise the minimum, but if they did you'd see it in their rules. c: I don't think I've ever seen an RP explicitly do that.
I see, I see... Most of the time there are activity expectations in an RP, so that if someone disappears they are automatically removed after not posting for a set time. You can usually find them in the rules of an RP.
You've got the basic idea down though, right? That's excellent~ The more popular RPs tend to go faster, though this is still forum RP so it'll never be as fast as say chatroom RPs.
I'm getting ahead of myself though. Is there any questions or concerns you have regarding the Roleplay Corner at the moment?