So all jfk trip would be max 3 days it wouldn't go in to the 4th day.
Er no, as I said
"first in, first out" happens the majority of the time but not always
, my emphasis...
There are
infrequent 4 day or even very very rarely 5 day JFKs. That might be due to a change in the 744/777 "ratio" on the route over a couple of days (e.g change from summer to winter timetable, or vice versa), or if some services are "dropped" on a particular day (as used to happen when very few people travelled over Christmas.. ).
Finally I'll belatedly I'll point out, because someone's bound to mention it, that there's also 2 day trip construction( early AM departure LHR, daylight flight out, night off, early AM departure JFK, daylight back, into LHR in the evening)