All storylines will have slightly different length. But if you "speedrun" the plotlines, most of them will come at around 3 months. Some of the plotlines are dependent on the "seasonal" events though, so depending on when an how the storyline progressed, that can stretch the plotlines somewhat, while you are waiting for the next opportune moment.
As for the "post-dating" content. I am planning on having one or two normal post dating events, and a couple of minor events to boot.