If you can start the engine without pressing the gas pedal down, which should be the case when it's hot, it doesn't use any extra fuel at all to restart an engine. On all but CV carburetors (usually only on foreign cars, like the 240-Z) if you pressed the gas pedal down the accelerator pump squirted more fuel into the carburetor. Normally when the engine was cold, you would press the gas two or three times which would both squirt more fuel into the intake and set the automatic choke, then you would start the car. This would be the case for American-made cars from the 50's and 60's. So if it could be restarted without pressing the gas pedal down at all, even 20 seconds would save fuel. Modern fuel injected engines would be even better, because they don't squirt in any extra fuel.