I have to disagree with Lawrence on this point. I think there is ample evidence in the Elric saga that demons, particularly guardian demons, could be bound for long periods or indefinitely. Theleb K'aarna's demon guarding Ashaneloon had obviously been there for some time -- days, if not months.
The demon that guards beggar-king Urish's treasure is in Nadsokor for several days, at least. More importantly, Elric makes it clear that this demon may be forced to remain guarding the beggar-king's treasure indefinitely:
From the Vanishing Tower, Book II, Chapter 4:
Elric bowed sardonically to the demon and returned his salute. "Farewell unlovely one. May your master not return to release you and thus ensure you squat in this filth forever!"
The demon slobbered in rage. "My master is Theleb K'aarna—one of the most powerful sorcerers amongst your kind."
Elric shook his head. "Not my kind. I shall be slaying him soon and you will be left there until I discover the means of destroying you."
Elric is as credible source on Young Kingdoms demonology that we can find in the saga. If he says that demons can be forced to guard something indefinitely, we should accept this as fact.
While I understand the issue of game-balance, I do not see a problem with allowing adventurers to bind guardian-demons for extended periods of time. The risks involved are significant. If adventurers want to sacrifice POW to have a demon bound to guard their treasure, I say let them. But the adventurer should word the guardian demon's instructions very carefully, or while the adventurer is off adventuring, tragedy may strike. What happens when the curious child of one of the adventurer's servants or friends wanders into the area the demon guards? Even if the demon doesn't attack the child (because of the adventurer's carefully worded instructions), the adventurer may return to find his or her home burned to the ground, and the city watch looking to question him about illegal sorcery.
-- Charlie