Thanks for the post.  We usually look at each listing throughout the listing period and evaluate how it is going on a case by case basis.  We usually chat with the seller throughout the listing process to make sure we are inline with the market value for the area, and if it looks like we are in that ballpark for pricing the home, we will extend the listing past the set listing period.  I would think the only reason not to continue a listing would be if the area that you are listing your home has taken a drastic decline in the market value of the home and the seller is unwilling to adjust their price accordingly or unable to adjust their price.  But, there really is not set practice in regards to this, it is more just a case by case basis for each listing.  Hopefully that helps and feel free to reply if you have anymore questions regarding this.  Thanks!