YMMV on this. I booked a cheap flight & cancelled. The remaining delta came back as a trip credit, with a date less than a year. I couldn't tie the date AA came up with to anything. It was 6-9 months in the future so served it's purpose, but did not produce a meaningful data point.

The luck I've consistently had, (knock on wood), with flight credit and trip credits, is purchasing a slightly higher priced flight. Then when canceling that flight, it's rolled over to a year out from the new purchase date. Sometimes as a new flight credit & sometimes as a trip credit.

I'm making it a point to use all my credits this year. It gets a little wonky with the multiple rollovers in the file. A capable AA agent can pull them up and it sometimes raises questions, with the ones that won't work online. It's not always straightforward trying to apply the new credit. Even though it's eventually worked out in all my cases, at some point my luck will run out.