It's entirely up to the GM. It could be daily posting, every 2-3 days, weekly, whatever.

The important thing a GM should do, and this goes for absolutely every GM and every single RP when they create it, is they should explicitly say what they expect the posting requirements to be. This allows everyone interested in the rp to be on the same page concerning this, and also makes sure any aspiring players for that rp understand and agree to commit to it. RPs that involve players who are happy to post more quickly then others, say once every two days, but who get bored/lose patience quickly thanks to players who only post once a week... it's not a good mix. Everyone should be able to know they're on the same page. It's better and healthier for the rp, it makes it more stable, and it brings more like minded players together, knowing what the expectations for the rp are. 

If you don't have a particular posting structure in mind, make this the first thing you ask for feedback for from the players and get a group consensus on it. This is equally good because already it's getting people to talk, discuss, and come to an agreement upon something. 

Personally in my rp, even though it's set in the casual section it basically runs at Advanced standards (2 paragraphs minimum, concise writing standards, focus on character development etc)- for the purposes of what I expect to achieve in my rp it's daily posting. However I'm not incredibly strict so as to always require that and I make allowances for players not to, particularly in cases of exams, sickness, holidays, and general breaks and offdays. Players are also encouraged to be communicative if they're unable to post for whatever reason. But otherwise daily posting is encouraged and expected in order to keep the rp going, to the pace I desire and help ensure the rp is remotely completable within a decade. Because I've stated this clearly and frequently enough, my players know what they're getting into when they join my rp, as well as understand the reasons for it and at the same time come to understand how it's a benefit and provides stability.

On the subject of length it depends on the scene and context within. I dislike gigantic reams of text that add little to the plot bar superfluous details, and honestly with a daily posting frequency it helps make sure posts aren't overly long because the reading time for that would be ridiculous. But again, it depends on what's occuring within the rp. My posts tend to be longer since I'm the GM and am controlling most of the characters and NPCs as well as setting up scenes etc, but even then I endeavour not to make them overly long. [url=http://www.roleplayerguild.com/topics/411/posts/ic?page=53]I'll just link a random page of my rp;[/url] you'll be able to see for yourself the lengths of players' posts and my own as well.