It varies between rps and is always a good thing to ask the GM beforehand. Minimum advance standards (2-3 paragraphs) isn't really difficult, but the massive length to which some posts can reach is a turn off for me at times. I think games which mark themselves as High Casual tend to normally expect 2-3 paragraphs per post but also limit the upper reaches of how long a post can be so it's not as absurdly long as some Advanced rps can reach, but again it varies. It also depends on the post in question and what there is context wise within the scene. Some scenes will require/inspire more writing, others less.