The pool holds about 8 TB on four disks. A scrub takes most of a night, and the stock schedule started it on the first Sunday of the month at midnight, which overlapped with whatever was still playing.
I disabled the packaged timer and added my own, Tuesdays at 02:30:
[Timer] OnCalendar=Tue *-*-* 02:30 Persistent=true
Scrubs now finish before anyone wakes up. Nothing clever, just a better hour.