Testing performed in-house has shown that an Outlook client (default configuration), on average, makes between 9 and 12 requests to the PingFederate STS endpoint per hour.
Additionally, the first time an editor is opened, a request is made to the PingFederate STS endpoint.
To help size your deployment performance results for the Username and Kerberos Token Processors are documented in the PingFederate Capacity Planning Guide and Performance Tuning Guide.