Hmm. I'll speak to my experience as a GM, and from stories I hear from other peoples being a GM. That was an odd sentence. Anyways.
When I make roleplays, the first thing I do is try to determine the availability of all the players. There are some folks out there who are just crazy in how much they can post. When constructing the roleplay, I try to make it
very clear that sometimes I don't post much at all. A week is generally my role, with four days if I'm feeling ambitious.
As for Kestral's point about removing players, I know someone currently GMing a
very expansive and involved storyline here on Gateway who was constantly frustrated by a player who just wouldn't post. It was putting everyone else behind, and they would keep skipping her turn. The character wasn't integral to the story, but if she
were I think there'd have to be talks about perhaps taking over the character for the sake of the story.
Another option for post frequency is to think of it less in terms of "let's get this many posts in this period of time" and more in terms of "take as many posts as you need to get to
x point in the plot."
For example, when I made my last roleplay,
YOU HAVE TO BURN THE ROPE, I designed it specifically to circumvent my problem of sometimes falling off the face of the Earth. My position as a GM was not integral to the active posting. I played the narrator, of sorts; a faceless voice that appears once everyone else (there were five other players) would post. The other players were highly active and would sometimes post two times a day.
I would appear at checkpoints. The players would have a goal to reach (in this case, to burn the rope) and within that goal, smaller ones - like choosing a weapon, reaching the end of a certain hallway. The players could take as many posts as they wanted, they could interact in any way that they wanted, as long as they ended up at that point - which is where I would reappear.
Just some thought!
-VV