I received a response via Ticket from Cipsoft because I don't think there's a way I could test this and give you an accurate answer. "The chance of the cleanse charm being triggered is 10% per attack received. Cleanse will remove one existing negative condition the character has when it's triggered, regardless if the condition was applied by the creature on which the charm is activated or not. If the character has more than one negative condition, the selection will be random. It is not limited by time. The immunity is granted until a new attack/damage with a negative condition is done."
Wanted Cipsoft to clarify what "not limited by time" means and I received this additional response:
"If you setup the "Cleanse Charm" on some creature, you have a 10% chance, one of your negative effects (which is already active) will disappear and your character is immune for 11 seconds to this effect. The "cleanse" charm is permanently active if you activated it. If your character have no negative effects on him, nothing will happen."