How much food gets wasted at the end of a (long haul) flight - I don't mean leftovers, I mean non-distributed meals?
Very hard to come up with a general figure, but it will depend on a combination of factors:
The airline's catering policy - some airlines will cater according to booked load, others will cater more. Some airlines will even cater below booked load on some flights, esp. red eye flights, because they can predict that a certain percentage of paxs will want to sleep and not eat. Obviously, the more you over cater, the more you waste.
Classes of service and meal service procedures. If you have First Class, it is likely to contribute to more wastage because airlines will generally not undercater First, and they are likely to overcater a little to ensure that all paxs get their preferred meals. Also, wastage will be more if the service policy is to plate the meals in front of paxs instead of serving them pre-plated.
The passenger no-show rate. The more noshows, the more meals wasted.