I should probably clarify this question a bit.
The two ways I see this could work are:
1. There is an ad count for each user (or it is calculated dynamically) and a the free ad counter is used to compare against the ad count and if the ad count exceeds the free ad counter limit, then additional ads are billable.
2. There is a means by which each ad placed is tracked against the user, and the user is allowed only one free ad. Of course this would imply that there would be some expiration to the time frame in which the limitation applies, and since I can't find that, I have to presume that number 1 above is how this is designed to work.
I am simply hoping to confirm this.