If you have to leave at a particular time (e.g. you're only planning to be online for 45 minutes), you can tell a student when they call in that you'll have to leave soon. That way, the student can be prepared to say goodbye or can call another tutor who is available for more time. You can also check the box in the top corner that says "I need to go after this call."
When you're on a Priority Hour, however, you are required to answer every call that comes in. If you know that you need to leave when your PH ends, you can select the checkbox that indicates that you have to leave after your Priority Hour. Once you select that checkbox, subsequent students will be informed that you'll only be online for a limited time.