What is the respawn time of the Orc (who drops the Old and Used Backpack) in Orc Fortress?

Question

How many days can pass before we see this little Orc?

Answer 1

It's a rare drop by one rare Orc in the Orc Fort; The backpack is red embroidered with the message "Property of Sam, Thais."
You can loot the backpack out of one orc inside of Ulderek's Rock.

Researchs says that the respwn time is always between 13-16 days from the last time it appears.