I don't think there is an average within any industry. It should depend on the objective of the emails and the target audience. E.g. If the subsequent email is a follow-up to non-responders of the first one, it may go within 2-7 days as most email responses should come within a week. If it is for lead nurturing with a long purchase lifecycle, you may have 30 to 45 days lag between more informational emails. We also have campaigns where we vary the wait time based on customer profiles and actions they take. In general, I find it a good practice to acknowledge explicit customer actions like filling out a form.