What do I mean by this?

How long do you think a roleplay should last, not OOC day wise but in terms of content.

Do you plan acts, sections? Does everything just flow freely, do you have a general idea of how things will go?

Well, I think it depends entirely on what the rp is about.

If it's some sandbox-esque SoL or fantasy adventure rp, then there's no real defined end. You sort of just go until it feels like there's nothing left to do, and then end things there, whether it's living happily ever after, or heroically sacrificing yourself for the greater good.

But if there's a defined goal or plot in mind, then it should last until that goal is met, such as rescuing the princess from the dragon, or find the lost Mayan artifact at the bottom of a tomb. How long this takes is entirely up to the players. If they decide to expand on the world, Interacting with NPCs, trying to solve other side problems, or just messing around for character development, it'll take a lot longer than people who just want to get to point A to point B.

There are advantages and disadvantages to each of these, of course, such as bordem or burnout, or feeling like the adventure was too 2-D and with little depth. I don't think there's a defined number of pages, words, or acts, it's however long the players decide the rp should go.

I personally am a fan of having a defined goal in mind. It gives us something to work towards so we don't end up standing around with our thumbs up our ass half the time wondering what to do next. If that goal ends up taking us only a short while, then so be it. If we decide to expand on the characters for a while, and it makes things take a little longer, then so be it.

I think it's a great idea to have some kind of general idea where you want the RP to go, regardless if the RP is sandbox, linear, or w/e. Obviously, it isn't as important in sandbox, but it's nice to have some sort of idea for those who would rather twiddle their thumbs and lump everything on the GM despite it being a sandbox RP. I'm not saying to literally railroad your players, as that would defeat the purpose of a sandbox RP, but that way you can give them a slight nudge rather than hear them go "But you didn't give me anything to write about!" (which has happened to me before).

Sections/acts/arcs are also a great thing to plan out as they give you an entire road map as to where you want it to go, but truthfully they're also useless because the site (and yes, I'm including myself here) is prone to falling out and/or ghosting RPs. You're putting in too much effort for something that'll fall apart usually in the 1st or 2nd page.

What do I mean by this?

I have no idea.

How long do you think a roleplay should last, not OOC day wise but in terms of content.

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 playing a videogame Vs making a story. I could have easily gotten together a small team and made 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.

Do you plan acts, sections?

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.

Does everything just flow freely, do you have a general idea of how things will go?

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.

I approach RPGs, both online and tabletop like I do writing fiction in general.
An RP should always have a defined 'goal' or 'end' that things are working toward, as every good story needs an ending, and the Player Characters are the agents of that story and their actions should drive it forward.
Of course, RP and RPGs are different from (most) other types of storytelling because the plot advancing is resting on the actions of the players as much as the 'author', the GM, who wrote the story.

'how long' it should last is a difficult question. I think it depends on how your plot is structured and what events you want to include. I tend to plan around smaller arcs that feed into a bigger plot, and that with the end of each arc, the players discover something that will help them get a grasp of the larger scale, and bring them more into the spotlight of events. Each arc is an adventure, focused around one or two major locations, and also usually spotlights one characters' background and their plot more than the others, so everyone gets a chance to be the 'star'. The arcs will also increase in the level of drama, tension, and higher stakes as the players advance through the plot.

Usually that's about 3-5 arcs and then a climax and a resolution/Epilogue/Denoument.
In tabletop terms, that's usually about 2-3 session per adventure, depending on the length of the session, and of course, the time OOC it takes depends wildly on how often you meet up to play.

Online, it depends how quickly the game moves, which is reliant hugely on how often people can post.

Roleplays should last as long as the participants wish it to.

Starting a story with an idea in mind of where to start and where to finish is good, but leave room for continuations (timeskips, spinoffs) if you're really feeling the chemistry with your partners, and wish to extend your partnership beyond the main story. I remember starting a story as a one-shot idea with a partner, but we loved our chemistry and the potential of the universe so much that we've decided to expand the story and created spinoffs within that same universe.