In Ingress, when are links destroyed?

I can bring you my experience on the field: the links on a portal are destroyed when there are two or less resonators remaining.

Sadly, the "official documentation" is more a collection of FAQs than an actual manual, so you'll have to rely on external resources or your direct experience to gather this kind of informations and become a better Agent.

After further testing, it appears that all links are dropped once there are only two resonators remaining. The energy left in the resonators does not seem to matter.

How far you can link form portal depends on the total energy of the portal.

From my observations, but no calculations:

The link goes down when the energy of the portal goes below energy needed to create that link.

When attacking a portal with multiple links they get destroyed consecutively, not all at once. This supports the above theory.

The link(s) are dropped once all resonators at the portal are at critical level.

manual source