Most Promote It campaigns are weekly recurring campaigns, which means the campaign will automatically renew each week until it is cancelled by the user. Promote A Show campaigns are one-time campaigns - they will only run the week of the show that is being promoted and then end (they will not automatically renew).
If you sign up for a free 3-day trial of Promote It, your campaign will run for free for 3-days, starting when your campaign is approved. You will receive an email to notify you that your campaign has been approved on the day your campaign begins running. After 3 days, if your campaign renewal has not been cancelled, your campaign will automatically convert to a paid weekly subscription (and will continue to auto renew on a weekly basis until the campaign is cancelled by the user).