Instead of focusing on timing and frequency, you may find it more valuable to consider what content you're sending in your follow-up emails. Are you simply nagging them to call you back? Or are you providing helpful content based on the needs/challenges you uncovered during the initial call? Put yourself in their shoes. What kind of email would you click on or respond to if you were in their position? If you're solely focused on being helpful and not asking anything of them, it won't matter if you send the email 2 days later or 5 days later. (Though I'd argue sooner is better than later, within reason.)

Hope this helps! Let me know if you'd like to discuss further.