In Toad 10.5, it's not possible to change the Polling Frequency in the DBMS Output Editor tab.
If the number is changed, and then the mouse focus is clicked elsewhere, it reverts back to the default 5 seconds Polling Frequency.
WORKAROUND:
Change the Polling Frequency and press ENTER.
STATUS:
Issue fixed in Toad 10.6 beta (and subsequent commercial release)
