This really has to do with the players' individual preferences. And it can change over the course of a scene.
If you write too much, you risk controlling the actions of the other character...and that can really annoy people. What I mean is, there's only so much you can write
your character doing before the other player says "Hey, my character would have done something while you were doing that!". Also, if what you write is
really long, people may be disinclined to read it...
On the other hand, if you write posts that are too short, the other player may become annoyed that you're not putting enough effort into it! They may feel like they're doing all the work and you're just being lazy.
So you see, it's a balance.
If you're wondering about
number of posts...Well, that has to do with the scenario. Some roleplays will come to an obvious conclusion. Some others you just go on for a while until someone says "Okay, this is getting repetitive so my character passes out". Really, it just depends on what you
want out of the roleplay...