The practice's working hours are set globally in Settings > Sites. You must have Level 4 (Administrator) permission to do this.
Click on the site, and then on the Calendar tab to see the calendar options, including the opening hours.
Tick or untick each day, according to when the practice is open and click on the opening-closing times to alter as needed.
Remember to click the Save button at the bottom of the screen to save your changes!
If a particular practitioner does not work your full practice opening hours, you will need to block the time they are not working using Recurring Appointments.
There is a web page to help you with setting up recurring appointments.