For each shorte.st's script type you are able to set up capping options. There are 2 values which can be adjusted:
limit - configures limit of ad impressions for user in a given time
time - configures how long above limit is taken into account. Unit is hours.
For example: Capping limit - 4, Capping timeout - 1 means that everyone will see the intermediate page maximum four times in one hour.