In case of a real nuclear attack, you'll probably go from DefCon 5 directly to DefCon 1. OTOH, since people at high levels are discussing the subject, you can move to DefCon 2 right now, but most of us can't stay in DC-2 very long, so we fall back to DC-5. IMO, in a real situation, you'll get 20 minutes warning tops. The missile with your hometown as its target will have already launched. Just sayin'...