I don't believe (based on experience) that there is one set cut-off time systemwide. It varies widely depending on the route, class or service, mid or transcon, int'l, etc..

Generally speaking, midcons the cutoff is 7:00 p.m. on those where Y receives meal service. In F, after 7:00 p.m. you will normally receive a snack.

There are a lot of midcon markets where AA doesn't serve a meal period (in Y) on flights of a certain duration, but does for the same flight time in other markets. DFW-LAX you will be fed in Y, but DFW-FLL never receives one even though they're both three hours. Of course, many of these are Bistro Bags so you'll have to determine if they constitute a "meal" or not.

Transcons it's a little later...8:00 p.m. in Y/F, and a snack in both classes after 8:00 p.m.

Int'l...DFW/MIA-MEX there is never anything in Y, and in F a meal or snack, appropriate to the time of day...it's same for most Central America and the Caribbean unless from New York.

Int'l to South America, Europe, and Asia you'll get dinner no matter the hour in both classes. The late MIA-LIM, for example, leaves at 11:45 p.m. so you end up eating "dinner" around 1:00 a.m.!

I'm sure I missed a few but this is a pretty good guide.