I guess so... :) But what is the schedule of the automatic monthly scrub, can I disable or change it manually?
It's part of the daily periodic that kicks off at 3am each day - it will perform a scrub of all pools when 30 days has elapsed since they were last scrubbed (it determines the date/time of the most recent scrub from "zpool history").
If you perform your own scrubs according to a more frequent interval (ie. weekly, which is recommended for non-Enterprise disks) then periodic will never initiate it's own scrub. If you leave it to periodic, it's harder to predict on what day the automatic scrub will kick off, and it's very likely to start on a weekday which may be highly undesirable if it takes more than 5-6 hours and FreeNAS is being used in an office environment as the scrub will over run into normal working hours.
There's no easy way to reconfigure periodic, other than to mess around with the default scripts which will then be overwritten with each firmware upgrade, so not really ideal. Better to schedule your own scrub - see
here for a script I use to schedule a weekly scrub (on Sunday morning) using cron.
How long it takes to do the scrub, as it has already ran about 5 hours?
Depends. A 4x 1TB disk RAIDZ1 volume with 2TB of used data takes 3 hours 45 mins on an AMD Neo (Intel Atom-class CPU) with 4GB RAM.