If the chalace had an ongoing effect then she would not have to make another check. Maybe while holding it she gets a bonus to will saves or something. So, after activating it, she doesn't have to keep checking. However, if it has an effect that must be activated, for example it can cast cure minor wounds, she has to use UMD every time it is activated.
So if you activate it, you don't have to activate it again until its done doing whatever its doing. After its done doing what it's doing, you have to use the skill again.