Is it customary for a student to reply to professor's email within 24 hours?

I got an email from professor M asking me if I will have time for a tea with her on Firday, and then I came up with this question:

Even if it is an important email from your professor (e.g. select your thesis or adviser), when you need some time to think before you make the final decision,

how fast should a student reply to the a professor's email before the student is considered a lazy student, or a student who tries to slack off in studies?

If I write the reply to my professor's email after 3 days, will she get angry or even unfriend me?