I have recently asked an Alternative Dispute Resolution agency to make a determination on a case that had me affected.
According to the adjudicator, my plane arrived at destination with a delay of 2h49m. My claim was that the doors of the plane opened more than 11 minutes later, and since I was familiar with the interpretation presented in @dunni's answer, I considered myself eligible for refund.
The adjudicator agreed that the time the doors are opened is what should be taken into account for calculating the flight delay.
As detailed in the case of C-452/13 Germanwings GmbH v Ronny Henning
the concept of arrival time for the purpose of EC261/2004 is when the first
door of the aircraft is opened.
However, at least in my case, there wasn't any available record of when the door opening took place, and since I couldn't provide a proof, they determined, on the basis of probability, that the doors were most likely opened before 11 minutes.
it is reasonable
to suggest that eleven minutes is sufficient time for the first door of the
aircraft to be opened.
The Passenger does not accept that the delay was less than 3 hours but has
not provided any evidence in support of his position.
In short, and based on a single experience, the arrival time is the time the doors are opened, BUT unless the passenger holds proof of when that time is, it's likely that the time of landing, plus a "fair" time for ground operations (apparently less than 11 minutes), is what will be considered by a Dispute Resolution agency.
