It depends. On Leolist you can re-use the same ad, you refresh the ad and it will take into account all the previous views. Some posters that are more cautious (or paranoid) will delete their posts after they are done working for the day and will create a new one and it will start from zero again. That's what I gather from my observations. If you refresh an ad page in your browser you can also see the view count increment by one.