Q:

For how long must a molecule remain stable to be considered "stable"?

In the Star Trek: Voyager episode The Omega Directive, Seven of Nine says that the Borg synthesized a molecule which was "kept [] stable for one trillionth of a nanosecond before it destabilized". That's $10^{-21}$ seconds, which is pretty brief.
Of course the point of that part of the script is to illustrate how unstable the molecule in question is. But this got me thinking: in our world, for how long must a molecule remain stable to be considered "stable"? Is there a generally applicable time limit or range (even order of magnitude)?
I did find Lifetimes of stable particles, but both the question and accepted answer talks about fundamental particles, whereas I am considering molecules.

A:

Molecules vibrate with frequencies in the range 10$^{12}$ to 10$^{14}$Hz. Although I don't know of any strict definition, I would take the view that a molecule must hold together for a few vibrations otherwise what you have is a collision not a molecule. That means the lifetime must be greater than 10$^{-14}$ to 10$^{-12}$ seconds, depending on the molecule.
To take Seven of Nine's claim, a vibrational period of 10$^{-21}$ seconds, i.e. $\nu$ = 10$^{21}$Hz, is an energy of around 4Mev, and this is far greater than any characteristic energy of molecules. It's more like the binding energy of nuclei. Perhaps Seven of Nine meant that they synthesised a new element that was stable for 10$^{-21}$ seconds.

