When using system 3 Fluorescent (fugitive), when will it fade and how much? Is there any medium?

All common fluorescent pigments are fugitive.

In direct sunlight fading can be noticed within a few days. Away from direct sunlight they may last for a few years. Initially the colours lose their brilliance, then fade away completely. They are not recommended for any professional or permanent art.

Glazes and Varnishes will hardly improve their stability.