All classified ads expire and will automatically be removed after 90 days - but this 90 day count starts from the LAST activity on your thread (topic). So if you need it to run longer, just reply to your own post and the day count is reset back to 0.
Edit : I didn't add this initially because sometimes it doesn't appear when it should, but you can also look for the "bump topic" link near the bottom left of the page. This will only appear on topics that you started.