I think that AS's policy is that they'll notify if there's a change of fifteen minutes or more. So far I haven't had a single change that wasn't more than five minutes, though.

Interestingly, I opened the Android app and got this notice for a couple of reservations last week--one on AS metal, one a partner award with an AS segment. I had to call in for the award res, but when I checked the desktop site the next day the notice was gone and my new departure time was right there.