How long should the Aurora's last?
I'd wager around 36 hours. It's hard to say for sure, depends on the properties of the plasma cloud; density, speed, and magnetic field orientation. Here's my favorite model. They're expecting it earlier than I've predicted, and if the interplanetary magnetic field is oriented northward, like they're showing, the conditions aren't ripe for magnetic reconnection, which severely diminishes the amount of geomagnetic storming. Still, our magnetosphere will constrict from the force of the impact, which will give us a significant substorm, if nothing else.
The latter. We can't really "see" the plasma in between the time it leaves the corona and arrives here. One of our best tools is the ACE spacecraft, which orbits around Lagrangian Point 1, about an hour upstream of us (under typical solar wind speed conditions). For a storm, we'll see it coming about 20 minutes before it hits the magnetosphere, but then it takes about another hour to propagate back through the magnetotail and send ions slamming down into the polar atmosphere... then voila, aurora.