By default, the Pega 7 Platform can maintain 1000 unique instances of a read-only data page per thread.
If the number of instances of a data page reaches 60% of the established limit, the system begins deleting older instances.
If the number reaches 100% of the established limit, the Pega 7 Platform deletes all data page instances that were last accessed more than ten minutes previously for that container.
You can change the instance limit by editing the Dynamic System Setting datapages/mrucapacity.
For more information, see PDN article, How PRPC manages data pages.