There's no way to know when an email has actually been Received as in "pushed or pulled to an end-user's Inbox."
The Delivered Email activity may seem to be a rough facsimile of this event, but it's far from exact. In some cases the first-hop mailserver will hold the mail for a brief time until a second-hop mailserver is available, or may forward it to a POP3 or IMAP4 server that may not be checked for days, if at all.
Similarly, the Sent Email activity doesn't represent when an email goes "out for delivery" (that is, is placed in the outbound SMTP queue) because internal conditions may mean it's still waiting to be queued. That said, you may actually want to start counting from the time when you put Send Email in a flow, so this would be an accurate measure.
In any case, there's no built-in field to reference this delta. You could use Trigger Campaigns and a Webhook to calculate the delta and store it in a field (though I would only do this for a sample of your leads each day so this side task doesn't consume too many resources).
It may also be possible to do this using RCE, if you have it. Dan Stevens would know if this is feasible.
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