Although "would" can be used for politeness, that behaviour is usually for requests and commands:
Do you want tea? / Would you like tea?
Come with me. / Would you please come with me?
Here, alternating with a future tense, it does not indicate politeness, as the above commenters agree. Instead, this is the more general purpose of the conditional: it indicates a hypothetical scenario.
So the future tense would indicate that it will definitely happen:
What time will that be?
Here, Jack has already agreed to meet Sally's parents, even though he has not explicitly said so. In fact, he assumes that she knows him well enough to count on his agreement and that she has already scheduled a time in advance!
But the conditional tense does not yet agree that it will happen:
What time would that be?
Here, Jack has not yet agreed to meet Sally's parents tomorrow. Instead, he implies that whether or not he can accept depends on what time she was planning to set the meeting for, because he might have a schedule conflict.
If you want to make him more or less polite, you can phrase the whole thing differently. Here are a few options (all of them assume that he is agreeing to come).
→ Okay. Did you have a time in mind?
→ What time would you like me to come? (Note: "would you like" is the conditional of politeness!)
→ What time works for you?
→ What time?
→ When do you want me there?
→ Lunch? Dinner?