P&A,
The time to slow the rotors is I think clear in the absence of changes to the controls - the rotational kinetic energy in the rotor, divided by the power required to maintain steady flight. That's a small number of seconds.
I think there's a good chance that a falling helicopter with low rotor speed would land skids down, as a shuttlecock or dandelion seed, due to drag from the blades.
As to whether it would "tumble", that's also possible, in the absence of angular momentum from the rotors, but the reports of "tumbling" come from inexperienced eyewitnesses. A shuttlecock effect would probably at some point cut in to stop any tumbling, and with 10-20s for descent from 1000ft that might be enough time to end up skids down even if it was "tumbling" at the start of the descent.