You can enter the maximum working hours per month or per week for each employee in the "employees" sector. Just go on the profile of an employee and click on "edit employee".
Here you can enter the maximum working hours per month or per week. Once the employee is assigned to more hours than the specified maximum number of hours, a warning is displayed. The actual number of hours can be seen next to his / her name in the schedule. By moving the mouse over the name you can see the weekly working hours and the expected hours in this specific month.