What they said...
You can delete a topic started by you if no one has replied to it. Once someone has replied, the only thing you can do is remove/edit the message.
You can delete replies at any time. There is no time limit. Look for the
delete icon, click and a window will popup where you confirm your username/password and then delete. If you're having trouble -- meaning you're not seeing the popup window, then you'll have to enable popups on the site (at least temporarily). Or you can contact me or wait to see if I catch it. Every now and then I'll spot dupes and delete one.