Can happen suddenly, in several ways.
In some organisms, extra eyes can easily appear. In the picture below, two extra eyes have fused to give a beetle a fully functional "third eye" in between the others.This was accomplished by knocking out a single gene which regulated the shape of the head. The amazing thing is that starting out with the wrong head shape, the rest of the self-organizing development works on top of that, building compound lenses and connecting neurons.
Another mechanism is seen in "Drosophilia", a fruit fly. We know that the flies' eyes and antenna develop from the same lump of about 23 cells, which split and differentiate into eye and antenna cells. A single mutation in the gene regulation cause all of them to develop to antennas, producing flies with no eyes and 4 antennas instead of two. Since antennas and eyes are so close in the development pathway, researchers speculate that they both evolved from the same organ. There may have been an organism with two "sense organs" on its head, then a mutation caused the animal to develop 4 sense organs. Through later mutations, the organs specialized into two eyes and two antennas.
So to answer your question: The extra eyes can appear suddenly, they don't need to gradually separate from the other eyes over several generations. If you are in need for an explanation, maybe a mutation in head or body shape caused extra eyes to be expressed (like for this beetle) or some organ that has a common origin with the eyes now get expressed as eyes (like in drosophilia)
If you want to read more, the term for developing organs in the wrong place is "ectopic organ". Eyes can even appear on a drosophilia's legs and wings, though in that case they're not properly wired up and nonfunctional.
I do not recommend a google image search. :-)