Java System Performance Statistics - Retention Period

I have just installed a NW 7.4 Portal. When I go to the System performance statistics tab, I get a nice report showing the last 48 hours of calls.

Now, I have a request (from someone who really loves this report) that I produce it weekly, with data for the last 7 days. I've looked at the System Performance Statistics in SAP Portal - Portal - SCN Wiki page, the NW7.4  System Performance Statistics - SAP NetWeaver Administrator - SAP Library pages, and the web in general, but I can't find where I change the retention period for this data.

Anyone know ?

thanks