How long can I keep a dough made with powdered skim milk outside the refrigerator at room temp. before it goes bad because of bacteria?
The ingredients (milk) don't matter. You are not storing milk, you are storing dough. So it behaves like dough, not like milk. 

Dough is full of yeast (obviously). It fills the ecological niche which would be taken up by pathogenic bacteria in other foods. So, no bacteria can take residence there, regardless of the presence of milk. Treat it like any other dough - you can keep it on the counter as long as you like. 

And in general, the shelf life of food is not calculated by the shelf life of its most perishable ingredient.