[quote=@DeadDrop]
What do I mean by this?
[/quote]

I have no idea.

[quote=@DeadDrop]
How long do you think a roleplay should last, not OOC day wise but in terms of content.
[/quote]

I find it really hard to separate the two. Generally speaking, most people can stick with an RP for a few months. If you're lucky, you might get a few years if you have your shit together and found dependable players. How much content you can move through an RP depends entirely on how long you can hold your player's interest. I wanted to run a danganronpa RP that was just as long as the games were. Thing is, stuff happens a lot faster when you're [i]playing[/i] a videogame Vs [i]making[/i] a story. I could have easily gotten together a small team and [i]made[/i] a fan game over the run time of the RP. That RP lasted 4-5 years. That was kind of long, and I don't think many people can stay excited about something for that long. 

But if you want a formula, I'd say use something like...

(Duration of RP in IRL days) X (Number of posts per player per week) / (Players steadily growing apathy) = Perfect amount of content!

How much content everyone wants to experience before moving onto another RP differs from player to player. 

[quote=@DeadDrop]
Do you plan acts, sections?
[/quote]

Not always at the start of the RP. I tend to think in very broad "strokes" like figuring out what the main "quest" is and what the "game play loop" will be. Then I populate the world with characters, but I don't worry about individual arcs until I'm sure I know what my players want/expect. It would be silly to try and force a romance plot on players that are more interested in making shit dead. It really depends on the RP. 

[quote=@DeadDrop]
Does everything just flow freely, do you have a general idea of how things will go?
[/quote]

I let things flow freely so that I have an idea of how things will go.

The opening acts of my RPs tend to be pretty open and I don't go out of my way to demolish players out of the gate. It's more a chance to observe their play style and see how they tick. Then I can use that to plan a bit further ahead. I also feel like most players are figuring out how to play their characters. Things like the nuances of their behavior aren't something I know until I play my characters for a bit. But I don't do much sandbox stuff. I imagine you'd leave things open for longer.