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When arriving at a busy airport, airliners may be stacked in holding patterns, increasing flight time. I know airliners are required to carry extra fuel in case of rerouting (including a safety margin).
Once in holding pattern, aircraft may be reordered (not as simple as a FIFO), thus some aircraft may stay longer in their holding pattern.
Is there a legal maximum time an aircraft can stay in an holding pattern? If needed, the question may be narrowed to EASA and FAA regulations.
No, there is no limit. Aircraft stay as long in holding patterns as they have fuel on board.
At least in the US, there isn't any maximum holding time. The FAA's ATC Orders 4-6-1(c) give instructions for controllers, and they're allowed to issue an "indefinite" delay:
When additional holding is expected at any other fix in your facility’s area, state the fix and your best estimate of the additional delay.
[...]
When holding is necessary, the phrase “delay indefinite” should be used when an accurate estimate of the delay time and the reason for the delay cannot immediately be determined; i.e., disabled aircraft on the runway, terminal or center sector saturation, weather below landing minimums, etc.
The final decision on how long a hold lasts is really made by the pilot, not the controller. If a hold is taking so long that fuel is running low, the pilot can always choose to divert to another airport or declare an emergency for priority handling. This question might also be helpful.
Actually that happened recently. For operational/load reasons, JFK needed to use tailwind landing runways, making them too short for the weight of an A380. The A380 asked controllers for a more suitable runway but they were unable to break the flow enough to allow that.
(I also wonder if this A380 pilot was conservative, and other A380s were landing while he orbited. Perhaps his particular plane was heavy.)
The A380 clearly communicated their time available for holding before they needed to divert. As that time grew closer, controllers said "ain't gonna happen" and they diverted.
Anyway, there you see the mechanism: the pilots plan a divert, and from that, they look at fuel and decide how long they can hold. They tell ATC that if it's helpful.
There is no time limit. The aircraft may be holding until the pilot requests a diversion, or declares an emergency, which they will do depending on fuel on board.
I know airliners are required to carry extra fuel in case of rerouting (including a safety margin).
Actually not really. The required fuel is calculated as fuel needed to fly to destination, abort approach there and fly to alternate, certain time holding at alternate, and final fuel reserve.
The contingency fuel, for cases of route changes and holding is at pilot discretion. These rules mean that once the contingency fuel is burnt, the aircraft shall divert to alternate unless it is clear they will be able to land soon.
The pilots are also supposed to declare fuel emergency when landing with the final fuel reserve still on board is no longer ensured with the distance left and any expected delay. And once that happens, the pilots can leave the hold at their discretion.
It should be noted that these days holding usually only occurs when there is some disruption like thunderstorms crossing the airport. In good conditions the flow control mostly takes care of releasing the aircraft for take-off so they arrive at approximately regular intervals and rate that the airport can handle.
