I saw much misunderstanding, in the discussion on the Malaysian Airlines thread about the shortest time needed for a transport jet to reach the surface from cruising altitude.
Formulas from elementary physics class are not useful. In all cases, and at every point in its trajectory, the jet's vertical velocity will be limited by aerodynamic forces.
Estimating the lower bound of descent time turns out to be quite simple: the airframe as a whole, or any detached part thereof, cannot attain a Mach number that exceeds one by more than a minute margin. Drag increases to such a great magnitude by Mach 1, that it easily matches aircraft weight plus engine thrust. Even to reach 1.0 Mach is very unlikely.
We know this on two grounds:
(1) In the early days of high-speed flight research, aerodynamicists designed shapes that were carefully optimized for minimum drag in the transsonic region, fabricated these shapes in solid steel (many times denser than any actual aircraft), and dropped them from high altitudes (20 to 30 thousand feet). It was very difficult for them to reach Mach 1 even in this condition.
(2) Some hair-raising incidents with transport jets in which gross upsets resulted in uncontrolled dives, showing maximum Mach numbers near 1 even with engine thrust pushing the jet into its dive. For example, in the 727 dive near Detroit (of Hoot Gibson fame), the maximum Mach number achieved was 0.96.
To find a lower bound for descent time, assume purely vertical motion (no "forward speed") at Mach 1. For the example of initial altitude of 35,000 ft, this comes to roughly 34 seconds.
However, starting from cruise condition, the jet would have an initial vertical speed of zero and forward speed in excess of Mach 0.8, so at least some seconds would be consumed in the acceleration of the velocity vector from horizontal to vertical -- this acceleration (again) controlled by aerodynamic forces.
Note 1: Even in a very severe accident, the actual descent time would likely be substantially longer than estimated by these simple assumptions.
Note 2: A transport jet has much lower drag than any broken part thereof, so the terminal velocities of components after an in-flight breakup can be expected to much less than Mach 1.