I would definitely prefer they do it as far out as possible at this point. (UA had posted early this morning that all Sunday cancellations had already been done.)
We should only expect they'll do cancellations as far out as there is no chance for government reopening (plus 1 to 2 days to normalize). No one should expect more than a few days notice unless there are clear signals being sent from Congress that there is an impasse that will result in a longer "guaranteed" shutdown window.