Of course we will try and give a sensible estimate of delay, and further than that, if we can see the stacks on radar and think the delay is Nil when the CCTV says 'Less than 10' then we will say so. However, officially less than 20 minutes is No Delay. Heathrow Director started a thread on R&N regarding BMI Fuel policy which touches on the issue of JAR requiring flight into congested airspace to take on extra fuel for just this reason. It makes interesting reading.
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"Take-off is optional, Landing is mandatory"