That's awesome that you were able to see it, change lenses and still get a great picture! I think the burn was supposed to last 1 minute? The telemetry data was out there weren't very public about timing, so I don't think many people knew when the final burn would occur.
I was watching the launch live (online) and got really into it. One of the cooler things I've seen in a while.
RE: SpaceX Falcon Heavy Third Burn from the Anza Borrego Desert, San Diego, California