Ready NAS 3312

Even which model NAS it's on is going to affect the time.  On a similar NAS, I added a 6th 4TB drive to a RAID5 and it took just under a day to re-sync. That's the closest thing I have.

You can do an MDADM check and get a good idea of how long it will take, perhaps initiated over a weekend or some time when access will be the lowest.  You have to do that from SSH with the command 

/usr/share/mdadm/checkarray -all.

Note that your RAID5 (I assume) and spare is not the best for data protection, as your data will be vulnerable to a second drive failure during the re-sync to bring the spare into the array.  RAID6 gives you dual failure protection with the same number of drives.

If you have multiple arrays and don't want to dedicate a second drive to redundnacy on each, then a single global spare offers some benefit (you don't have to be present to swap/add a drive to start the re-sync).  But with a single volume, I don't see the benefit of RAID5 and a spare over RAID6 other than RAID5 is a bit faster (including re-sync for the same size array).

I have a 12-bay NAS with two RAID5 arrays and another with a single RAID6, and my access to both is about the same -- my network connection is the bottleneck.  That might be a bit different with multiple users accessing simultaneously.