average email opt-in rates for online retailers

I know this really is 'how long is a piece of hanging rope', bur can anyone share a figure on what would be an expected opt-in rate for an email newsletter sent out by a top-level online retailer (think Tesco or John Lewis)? It's for the web-based opt-in collection only, assuming that all  best practice in this space is followed (a well written opt-in clause and privacy policy, an preview of a newsletter available before signing up etc). Many thanks.

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