Converting stand mixing time to hand mixing For each minute of using a stand mixer on high, approximately how long (or how much effort, quantified) should a mixture be mixed (whisked/beaten) by hand to achieve similar results? 

Baking recipes are frequently encountered which only supply instructions requiring  use of an electric stand mixer. Sometimes these recipes indicate that mixing by hand is possible - and indeed, that used to be the main way mixing was performed.

This question inquires as to mixing/beating batters, specifically; such as cake or gluten-free batters - not kneading.
I remember when cake mixes included the number of strokes needed to mix the batter by hand.  The Betty Crocker FAQ website (a U.S. baking mix company) suggests 150 strokes per each minute of electric mixer time recommended.  Note: that is not 150 strokes per minute!  So if the directions call for 2 minutes of mixing, that translates to 300 strokes.

Betty Crocker FAQ Site

EDIT

The stroke count in this answer is specifically for use with a cake mix.  As KatieK pointed out, mixing time and strokes are really dependent on the ingredients and desired consistency for that particular recipe and step in the recipe.