OPP of "0" means no protection (though as indicated elsewhere, this can lead to really strange results sometimes - hence the 1 hour OPP suggestion)
OPP of "Infinite - Never Overwrite" is the other extreme
does the data on your disk include GRT data? if so, regardless of your OPP settings, that data will not be overwritten until the disk fills up or the Disk Space Reserve has been met. This is a design decision by Symantec, and, I think, a silly one
If/When the OPP expires, the BKF files will not be purged/deleted, their contents will be replaced. (think of eack BKF file as a tape volume )