What determines how often the pre-filters in a fume extractor need to be replaced?

This depends on how much particulate the process generates, the material being lasered, and the size, type, and grade of the pre-filter. For example, if a shop regularly lasers MDF, then the main emissions are wood fiber and resin/glue, which block a pre-filter quickly. If a shop lasers acrylic (PMMA), then most of what it gives off is gas, which passes through the pre-filter without blocking it.

   —BOFA International