The longer posts don't bother me, but I don't generally sell, I shop, so I happily scroll up and down. But I see the point of not liking your ad pushed out of the window.
However.
I don't think it's a good idea to hard code the time separations. People often ask for price checks, and then someone answers, and then the original checker might give a clarification or thank you, and it will suck to have to wait five minutes to do that.