It differs. Some stories are just going from one sex scene to another, while others can go for pages without so much as an uncovered breast. You should check on the description of starting games. Don't hesitate to ask if it remains a little vague.
Now, if you want to start your own story, you should check out the boards under Gaming and Writing to the left, namely "One on One / Solos Wanted", "Players Wanted" and "GMs/Games Wanted" (and Chat Help and Games if you're also interested in realtime games). One on One / Solos is for a game between two players, Players wanted is for a game between more than two players, and GMs/Games Wanted is for either two or more players, where the thread starter has an idea (s)he wants to play out, but is looking for another person to take upon the role of storyteller and lead the game.
Feel free to browse through the existing threads to see if there's a game of your liking. If something interests you, simply reply to the thread, or send a PM to the thread starter. It never hurts to inquire. You can also post your own thread with your own story ideas. Just give a desciption of the story you have in mind, as well as what players you are looking for. You might want to include what board you would like to play it on, as it gives a bit of info to the other players about the sexual level.
In addition, you might want to check the Ons and Offs threads (under General Sexiness), and make a thread for yourself. It's ok if it's small at the start, you can always come back and edit it to add topics later on.