The cooldown is 30 days. I found my first Galthen's Satchel on 23/07 and the second on 22/08. Before this date the tree was empty. This is similar to the Dragon Throne from Zao and it might be because of this that they can't be sold at market, like Zaoan Chess Box and Backpack of Holding.
I've tested with a friend and no need to fight Megasylvan Yselda to get another bag. The Artefact Box also come inside.
It's free items!!!