The current surge at startup is about 1 second long, give or take. So, it uses 5 or 6 seconds worth of FULL LOAD energy. But the current draw at idle is probably on the order of 1/3 the full load amperage. So, strictly from an energy consumption view, the break-even point should be somewhere around 15-20 seconds, give or take.
However, we haven't considered the wear and tear on the motor. The motor starter is a wear item and will eventually need replacing based on several factors including the number of starts. Dollars and cents wise, this pushes the break-even point out some more, maybe to 30 seconds or a minute give or take.
However, Safety trumps all of these! I turn my machines off because the horizontal surfaces are too inviting. I'm afraid I'll lay something down on top of a spinning tablesaw blade, or jam something into a router bit. And, I find I think better without the machines running in the background. If they are running, I'm sort of rushed to get back to them. I'll keep turning mine off regardless of the cost.
“We should be careful to get out of an experience only the wisdom that is in it and stop there lest we be like the cat that sits down on a hot stove lid. She will never sit down on a hot stove lid again and that is well but also she will never sit down on a cold one anymore.” - Mark Twain