Owed from two hours, whatever caused the delay, with no cap.
Check your flightDuty of care sits in Article 9 and it is completely separate from compensation. It starts at:
Once you are past it the airline owes you meals and refreshments in reasonable proportion to the wait, two phone calls or emails, and if you are delayed overnight, a hotel room and transport between the airport and the hotel.
It applies no matter what caused the delay. Bad weather, an air traffic control strike, a closed airport: none of it removes the duty of care. Those causes remove your right to compensation, which is a different thing entirely.
This is exactly backwards from what most passengers assume, and it is why so many people sleep on airport floors during major disruption while holding a right to a hotel room.
And there is no cap. In McDonagh v Ryanair (C-12/11), arising from the volcanic ash closures, the Court of Justice held that the obligation to provide care has no financial limit and no time limit, even in extraordinary circumstances lasting days. The airline argued that was unreasonable. The Court disagreed.
Which is common, because duty of care bites hardest exactly when the airline is busiest and least able to cope.
Reimbursement of expenses is a much easier argument to win than compensation, because there is no extraordinary circumstances defence to it at all.
Check if you are also owed compensation