Indefinitely. Essentially, if the material adheres to the screw, it can degrade and become baked onto the surface of the screw. I have seen this with materials and additives such as colorants. This is one of the reasons you should take great care of the polymer during screw recovery to promote smooth material conveyance and reduce excessive back-flow and stagnation.

Although there are many advances in purging compounds over the years, the most effective method of cleaning the screw is still to actually remove the screw and manually clean the screw surface.

-Andy