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It all depends on the car for one, and the place you do it on, the time spent coasting I/O is critical, as is consistency, I say at least 20 but really a 30 second glide on a regular basis considerably increases mpg.
There's absolutely no way otherwise, an ICE consumes more than the usual amount of fuel upon starting, even a hot engine injects additional raw fuel into the cylinders as a boost for this process. So a short glide doesn't do anything at all, too short a glide could easily hurt mpg.
Best places to do it is right as soon as I realize I am more than likely going to have to come to a complete stop, such as highly trafficked busy intersection.
Or when pulling into one's driveway, but it depends how the road curves and the hills and other stuff. My parent's driveway I can turn the engine off two blocks before and coast the whole way, but my own it's all uphill so it doesn't work... However, starting out in the morning I can coast out of my driveway and down to the stop sign before even starting the engine, again this varies from one house to the next.
That, or long down hills but at least 1/4 mile long or better.
Speed is important, it makes little sense to lose 20 mph for no good reason if one does not have to stop.
Also works better in a manual IMO, where a miscalculation simply translates to popping the clutch.
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