In my experiments, a Craigslist Ad takes up to 1 day to show up in the Google SERPs once it has been posted in a major market, and it lasts up to 3 days at the top of the results before it disappears.  For example, a recent post for a product on Thursday afternoon was on the SERPs by Friday Morning and stayed in the top 3 for searches on that product until Monday morning.

If this continues in our future tests, then we  only need to repost this ad every 4 days.