I find that if you don't "Add" them to your account (unless/until you need them), they often just "magically" extend themselves. Obviously, at some point they can go away, but I generally don't add them to my card until I have a specific/intended use. And as said, I find they just sort of keep getting extended if I don't take advantage immediately. However, once you add them to your card they do tend to expire on schedule.

Regards