You could test it with Google Adwords. Put up ads for "10 minute mail", "5 minute mail", "30 minute mail", and "42 minute mail", and see which one gets the most

You could test it with Google Adwords. Put up ads for "10 minute mail", "5 minute mail", "30 minute mail", and "42 minute mail", and see which one gets the most clicks. I heard Tim Ferriss claim that this is how he picked the title for his book, "The Four-Hour Workweek". On the other hand, you could also just pull 10 minutes out of the air and say "that sounds about right". Which it does.