What will really happen when the Thalmor eventually disable all of the Towers of Mundus? Will there be no more concept of time? Would time just end and things would seem the same? Or would there be some big explosion where the whole world morphs into a new world?

We're... not entirely sure.

WHat's most likely to happen, though, is that Nirn will... dissolve, for lack of a better term. From my understanding, the Towers are like physical pins, keeping the fabric of Mundus together. Once the last tower goes, Mundus would unravel, and return to a pre-creation state.

That is the ultimate goal of the Thalmor. And they're going after multiple targets; Talos to weaken the nine and cause Mundus to unravel, the towers to cause Mundus to unravel, and probably another, seperate thing that would end up causing Mundus to Unravel.

It's never been really defined. Because it's never happened before.

When the Thalmor disable all the Towers?