Is it customary for a student to reply to professor's email within 24 hours?
There isn't really anything special about academia in this regard.  If someone sends you an email that needs a response, it is always courteous to respond as quickly as you are able.  It doesn't matter if it's to a professor or a student or a friend.

Anyone might get annoyed with you if you take a long time to respond to an email to which they need a quick response - not just professors!  But there's no magic number - you can't say that 71 hours is fine but 73 hours is excessive.  Some messages are urgent and need a quick response, others are not.  If someone is expecting a quick response but you need more time to make a decision, you can certainly contact them to let them know you are considering it, and ask when they need your answer.

As a general note: several of your recent questions seem to be written from the assumption that academia has its own special unwritten laws for everything, and if you can only find out what they are and obey them, everything will be fine.  For better or for worse, that's not generally the case - there are no such objective standards.  The key to successful interactions, in academia or any other job, is "act like a decent and professional human being".