Is it permissible for me to attend three days a week, from 8:30 AM to 7:30 PM, and leave if there is no work, knowing that work often starts late and there is no work in the evening?

One must adhere to the official working hours stipulated in the regulations. It is not permissible to change them or leave before the end of the designated time, even if there is no work, unless the regulations allow for it. Being done with one's tasks is not a justification for leaving before the end of working hours in violation of the regulations. The fundamental principle is the obligation to fulfill contracts, as Allah Almighty says: "O you who have believed, fulfill [all] contracts" (Al-Ma'idah: 1). The implication of a private employment contract is that the benefit derived from the private employee is owned solely by the employer throughout the contract period. Thus, the private employee is one whose benefit is measured by time, and no one else shares in their benefit during the period of employment.