I remember reading a very unique book some 15-20 years ago. It was a fantasy story where the reader would get to choose what to do next at the end of each chapter. There were choices that lead to the death of the character so it was like an RPG that you had to beat in the form of a book.

Maybe giving the readers a choice that is determined by the majority of votes would let you plan the story ahead a little more than simply doing it on the fly. Unless, of course, you want to do it on the fly. But the overall idea of interactivity and the possibility to influence the way the story evolves seems enticing to me (Think TellTale games, for example).

As to the length, I think it's either up to you or depends on the amount of active followers. Something like a monthly event, maybe?