Posts erode depending on the number of posts in the streamer & how many "likes" or "dislikes" it has - so posts can erode in as little as a couple hours to a few days! Obviously if you delete your post, it's erodes immediately.
"Likes" adds time to a post (unless the poster sets a time limit, of course).
"Dislikes" erode posts.
Anonymous posts erode more quickly.