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Just my 2 bits.
As others have indicated a specific number will vary depending on conditions.
If you wanted a specific number ... you would have to define the specific BSFC for the engine in question and all the other driving context conditions ... can be done if you are that interested ... but different vehicles will have different numbers under the same conditions ... and the same vehicle will have different numbers under different conditions.
As a crude General Rule ~10 Seconds is a safe point ... but I think learning the methodology itself is a better method than trying to pick one specific number.
Properly used engine off coasting is an FE asset ... over used it is a FE negative... I'm not just referring to too short of a engine off period... too long of an engine off period is also a FE negative.
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I usually recommend those interested first learn Conservation of Momentum , and driving awareness ... leaving engine off coasting for later development.
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Then if they still want to go even further ... I recommend the BSFC chart for their engine ... and to make an effort to operate the engine more often in it's higher efficiency points.
If current driving conditions warrant it using a engine off coasting event can be an effective method of avoiding those less efficient parts of the BSFC.
An additional Benefit of Engine off coasting beyond just avoiding low BSFC points , is that it also avoids pumping losses ... something the Engine has to spend energy/fuel for even at it's best BSFC.
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The too long Engine Off Coast aspect is a factor of the exponential influence of aerodynamics ... meaning it is more aerodynamically energy efficient to travel at a steady average speed than it is to achieve the same average speed while fluctuating the speed up and down... no matter what average speed you want to travel at.
So there is a point where further engine off coasting will actually produce worse FE and lower MPG.
Although I suspect traffic conditions would often make it unsafe to fluctuate one's speed that much before you reached the FE loss point.
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