FWIW, on a 4 x 8TB ZFS RaidZ1, I run a ZFS scrub every night at 2:30am. if I had more disks in a raidz2 (equiv to raid6) then I might do it weekly rather than daily, but since a raidz1 is only singly redundant, I figure daily scrubs increases the chance I'll catch a failing drive sooner than than later