Is it permissible for a woman who constantly experiences discharge to perform ablution half an hour before the prayer time if she is going out and it will be difficult for her to perform ablution later, and how can she ascertain whether this discharge is continuous or not, and must she change the pad she uses for each prayer?

Vaginal discharge is pure but nullifies ablution (Wudu'). If this discharge is continuous, such that it does not stop for a known period sufficient for purification and prayer, then you are afflicted with سلس ( سلس البول ). The majority of scholars hold that a person afflicted with سلس must perform Wudu' for each prayer after its time has begun. However, if you find great difficulty in this, it is permissible to adopt the view of the Malikis, who state that the ablution of a person afflicted with سلس is not nullified by their continuous discharge. Consequently, they are not required to perform Wudu' for every prayer unless their ablution is nullified by another factor.