You do not need to re-apply glass separator to the shelf until you can see cracks and flakes in the glass separator. If you fire to medium fuse instead of a flat fuse, you can usually get many firings out of one application of glass separator. You should never fire glass on flaking or cracked glass separator. This can cause glass breakage or bubbles.
If you fire to a full fuse, recoat the shelf with new glass separator (also called shelf primer). However, if you are fusing to lower temperatures and the glass separator is not flaking off onto the glass, you shouldn't need to recoat the shelf each time. When fusing to a "tack fuse" or slightly hotter, you can reuse the glass separator many times. (This is the stage of fusing where stringers on the glass surface are still distinct and have not melted flat into the glass.)
Of course, this may depend on the type of glass separator you are using.
Shine a sidelight across the shelf surface. Or hold the shelf vertically under a light source. With light shining directly across the shelf, it is easier to see cracks and flakes in the glass separator. When you see imperfections, reapply the glass separator. If you have difficulty seeing the surface of the glass separator, play it safe by applying a fresh coat with each firing.
Use a putty knife to scrape off the glass separator. Hold the knife at a 45-degree angle. Removing glass separator creates dust, so you should do this outside. A putty knife will also remove ceramic glaze or glass that has stuck to the shelf.