If you respond to your post more than a day later, chances are nobody is going to make a fuss. If you respond to your post every 5-30 minutes, then it becomes an issue.
If you respond to your post on selling an item within about 24 hours, or just saying "look at my chicks" that can be a problem, while if you respond to an emergency post 2-3 hours after you posted, and it really is an emergency like injury where time is important it can be excusable.
It is a very fine line to what is acceptable and what is not.
The problem of reposts isn't so much a repost a few days later if your first post rolled to the bottom, but largly the same question that is posted in every forum sub section, or the same poster posting the same question multiple times under different titles.
Once again it is kind of a fine line. If you can find your old post on the first two or three pages, chances are, you probably shouldn't be dong a repost yet if you haven't had a response. If it's on the 4th or greater page, nobody's going to notice it if you post again and the same question wont be cluttering up the first few pages.