All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad, is His slave and Messenger.
As long as this woman carries out the work duties required of her, her salary is lawful, whether it is fixed or calculated according to the number of working hours.
If she is negligent in performing the work required of her, then a portion of her salary that is proportionate to her negligence is haram to her, unless the employer pardons her negligence.
An employee should be diligent in performing his work to the best of his ability while being cautious of laxness and neglect as well as of immoderation and obsessiveness.
Allah Knows best.