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.