You should follow up a lead as soon as possible! Or within 15 minutes of receiving an alert about an open email. 

It's imperative to capture your prospect's attention while they are focused on your message in the email. 

The Harvard Business Review found that 26.1 % of leads are followed-up with within 5 minutes, while the average response time for all leads is 42 hours. It's no secret that 35-50% of sales go to the vendor that responds first.  
Source: Incept Grows                   

The odds of making a successful contact with a lead are 100 times greater when a contact attempt occurs within 5 minutes, compared to 30 minutes after the lead was submitted.   
Source: LeadResponseManagement

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