The 24-hour rule is a guideline that suggests that you should wait at least 24 hours before sending a follow-up email after an initial email has gone unanswered.
The idea behind the 24-hour rule is that it allows the recipient sufficient time to respond to the initial email, and it also gives you time to think about your follow-up message and ensure that it is well-written and appropriate. While the 24-hour rule is not hard and fast, it can be a helpful guideline to follow to avoid coming across as pushy or impatient in your follow-up emails. It is always a good idea to be mindful of the recipient's time and to be respectful of their workload and schedule. So, if you don't receive a response to your initial email within 24 hours, it is generally a good idea to wait at least a day before sending a follow-up email.