fwiw, when I've observed it (mostly at SFO), ground stops on arrival start when the President's plane is roughly on final approach and end when the President leaves the airport. It's usually a pretty short period of time for arrival (like 15 or 20 minutes).

On departure, it's the same rule--starts when the President arrives at the airport and ends very shortly after he departs--but the departure logistics seem to take a bit longer (like 30 minutes).

I think it's smart to move your onward flight though. Even a 15 minute ground stop would seriously risk your connection. You may get a pretty good view of Air Force One though--at least at SFO they operated the airport up to the last possible minute and then just had planes stop mid-taxi.